It's an easy answer. Recall that Oliver Cowdery was ordained as the Assistant President of the Church in December, 1834. The notes from the meeting describe the significance of that calling this way:
"The office of Assistant President is to assist in presiding over the whole Church, and to officiate in the absence of the President, according to his rank and appointment, viz: President Cowdery, first; President Rigdon Second, and President Williams Third, as they were severally called. The office of this priesthood is also to act as spokesman, taking Aaron for an example. The virtue of the above priesthood is to hold the keys of the kingdom of heaven or of the Church militant."
Oliver, the spokesman designated by the Lord, would naturally be the one to write important doctrinal and historical matters, such as the eight historical letters, including Letter VII. Joseph assisted him, but it was up to Oliver to do the writing. For Letter VII, it didn't matter which of them wrote the words anyway; both Joseph and Oliver had visited Mormon's depository of Nephite records right inside the Hill Cumorah in New York.
Because of his responsibility as spokesman, it was Oliver, not Joseph, who gave the first Quorum of the Twelve Apostles an explanation of their calling and a Charge on how to fulfill it. Oliver ordained all of the Apostles. He was assisted in some cases by Joseph and Sidney Rigdon.
He gave the charge shortly after the Apostles were ordained.
Think about this the next time some intellectual tells you that Oliver was merely an ignorant speculator who misled the Church about Cumorah when he wrote Letter VII.
You can read the charge in the Joseph Smith Papers here: http://www.josephsmithpapers.org/paper-summary/minutes-and-blessings-21-february-1835/5.
The following Charge was given to the Twelve by President O. Cowdery.
Dear Brethren, previously to delivering the charge I shall read a part of a revelation. It is known to you that previous to the organizing of this church in 1830 The Lord gave revelations or the Church could not have been organized. The people of this Church were weak in faith compared with the ancients.
Those who embarked in this cause, were desirous to know how the work was to be conducted. They read many things in the Book of Mormon, concerning their duty and the way the great work ought to be done; but the minds of men are so constructed, that they will not believe without a testimony of seeing or hearing. The Lord gave us a revelation, that in process of time there should be Twelve chosen to preach his gospel to Jew & Gentile. Our minds have been on a constant stretch to find who these Twelve were. [p. 158]
When the time should come, we could not tell, but we sought the Lord by fasting and prayer, to have our lives prolonged to see this day, to see you, and to take a retrospect of the difficulties through which we have passed. but, having seen the day, it becomes my duty to deliver to you a charge. and first, a few remarks respecting your ministry.
You have many Revelations put into your hands. Revelations to make you acquainted with the nature of your Mission. You will have difficulties by reason of your visiting all the nations of the world. You will need wisdom in a tenfold proportion to what you have ever had. You will have to combat all the prejudices of all nations.
He then read Revelation, and proceeded to say, Have you desired this ministry with all your hearts, If you have desired it, you are called of God not of man, to go into all the world. He read again, from the Revelation, what the Lord said to the 12 Brethren, you have your duty presented in this revelation. You have been ordained to the Holy Priesthood, You have received it from those who had their power and authority from an angel.—
[Note: they were all ordained by Oliver, with Joseph and David Whitmer participating in some cases.]
You are to preach the gospel to every nation. should you in the least degree, come short of your duty, great will be your condemnation. for the greater the calling, the greater the transgression. I, therefore, warn you to cultivate great humility, for I know the pride of the human heart. Beware, lest the flatterers of the world lift you up. Beware lest your affections are captivated by worldly objects. Let your ministry be first.
Remember the souls of men are committed to your charge, and if you mind your calling you shall always prosper. You have been indebted to other men in the first instance for evidence, on that you have acted. But <it> is necessary that you receive a testimony from Heaven for [p. 159] yourselves, so that you can bear testimony to the truth of the Book of Mormon.
And that you have seen the face of God: that is more than the testimony of an Angel. When the proper time arrives, you shall be able to bear this testimony to the world. When you bear testimony that you have seen God. This testimony God will never suffer to fall, but will bear you out. although many will not give heed, yet others will. You will, therefore, see the necessity of getting this testimony from Heaven. Never cease striving until you have seen God, face to face.
Strengthen your faith, cast off your doubts, your sins and all your unbelief and nothing can prevent you from coming to God. your ordination is not full and complete till God has laid his hand upon you. We require as much to qualify us as did those who have gone before us. God is the same. If the Saviour in former days laid his hands on his deciples. Why not in the latter Days.
With regard to superiority I must make a few remarks. The ancient Apostles sought to be great. but, brethren, lest the seeds of discord be sown in this matter, understand particularly the voice of the spirit on this occasion. God does not love you better or more than others. You are to contend for the faith once deliverd to the saints. Jacob, you know wrestled till he obtained. It was by fervent prayer and diligent search that you have obtained the testimony you are now able to bear. You are as one, you are equal in bearing the keys of the kingdom to all nations. You are called to preach the gospel of the son of God to the nations of the earth. [p. 160]
It is the will of your Heavenly Father that you proclaim his gospel to the ends of the earth. and the Islands of the sea. Be zealous to save souls. The soul of one man is as precious as the soul of another. You are to bear this message to those who consider themselves wise. and such may persecute you, they may seek your life. The adversary has always sought the life of the servants of God. You are, therefore, to be prepared at all times to make a sacrafice of your lives, should God require them in the advancment and building up of his cause.
Murmur not at God. Be always prayerful, be always watchful. You will bear with me while I relieve the feelings of my heart. We shall not see another day like this. The time has fully come; The voice of the spirit has come to set these men apart. You will see the time when you will desire to see such a day as this, and you will not see it. Every heart wishes you peace & prosperity. but the scene, with you, will inevitably change. Let no man take your bishopric, and beware that you lose not your crowns. It will require your whole souls. It will require courage like Enoch’s.
The time is near when you will be in the midst of congregations, who will gnash their teeth upon you. This gospel must roll and will roll till it fill the whole Earth. Did I say congregations would gnash upon you, yea I say nations will gnash upon you. You will be considered the worst of Men. Be not discouraged at this, When God pours out his Spirit, the enemy will rage, but, God, remember is on your right hand and on your left. A man, though he may be considered the worst, has joy who is conscious that he pleases God.
The lives of those who proclaim the true gospel will be in danger; this has been the case ever since the days of righteous Abel. The same opposition has been manifest whenever [p. 161] men came forward to publish the gospel. The time is coming when you will be considered the worst by many & by some the best of men. The time is coming when you will be perfectly familiar with the things of God. This testimony will make those who do not believe your testimony, seek your lives. But their there are whole nations, who will receive your testimony. They will call you good men. Be not lifted up when you are called good men. Remember you are young men, and you shall be spared, I include the other three. Bear them in mind in your prayers carry their cases to a throne of grace. Although they are not present, yet you and they are equal. This appointment is calculated to create an affection in you, for each other, stronger than death.
You will travel to other Nations, Bear each other in mind. If one or more is cast into prisons, let the others pray for him, and deliver him by their prayers. Your lives shall be in great jeopardy, but the promise of God, is that you shall be delivered. Remember you are not to go to other nations, till you receive your endowment. Tarry at Kirtland until you are endowed with power from on high. You need a fountain of wisdom, knowledge, and intelligence such as you never had.
Relative to the endowment, I make a remark or two, that there be no mistake. The world cannot receive the things of God. He can endow you without wor[l]dly pomp or great parade. He can give you that wisdom, that intelligence and that power which characterized the ancient Saints and now characterizes the inhabitants of the upper world. The greatness of your commission, consists in this; you are to hold the keys of this ministry. You [p. 162] are to go to the nations afar off; nations that sit in darkness. The [day] is coming when the work of God must be done. Israel shall be gathered, The seed of Jacob shall be gathered from their long dispersion. There will be a feast to Israel, the elect of God.
It is a sorrowful tale but, the gospel must be preached and his <God’s> ministers be rejected. but where can Israel be found, and receive your testimony, and not rejoice? No where. The Prophecies are full of great things that are to take place in the last days. After the Elect is gathered out, destruction shall come on the inhabitants of the Earth; All nations shall feel the wrath of God, after they have been warned by the saints of the Most High. If you will not warn them others will and you will lose your crowns. You must prepare your minds to bid a long farewell to Kirtland. even till the great day come. You will see what you never expected to see. You will need the mind of Enoch or Elijah & the faith of the brother of Jared.
You must be prepared to walk by faith, however, appalling the prospect to human view. You, and each of you should feel the force of the imperious mandate, “Son go labor in my vineyard” and cheerfully receive what comes, but in the end you will stand while others will fall. You have read in the Revelation concerning ordination. Beware how you ordain, for all nations are not like this nation. They will willingly receive the ordinances at your hand to put you out of the way.
There will be times, when nothing but the angels of God can deliver you out of their hand. We appeal to your intelligence, we appeal to your understanding, that we have so far discharged our duty to you. We consider it one of the greatest condescentions of our Heavenly Father in pointing you out to us. You will be stewards over this ministry. [p. 163]
We have work to do, that no other men can do. You must proclaim the Gospel in its simplicity and purity. and we commend you to God and the word of his grace. You have our best wishes, you have our most fervent prayers, that you may be able to bear this testimony, that you have seen the face of God. Therefore, call upon him in faith and mighty prayer, till you prevail, for it <is> your duty and your privelege to bear such testimony for yourselves.
We now exhort you to be faithful to fulfil your calling, there must be no lack here. You must fulfil in all things, and permit is us to repeat, all nations have a claim on you. You are bound together as the three witnesses were. You, notwithstanding can part & meet & meet and part again till your heads are silvered o[v]er with age.
He then took them separately by the hand and said, Do you with full purpose of heart take part in this ministry, to proclaim the gospel with all diligence with these your brethren, according to the tenor and intent of the charge you have received, each of which answered in the affirmative.