Metzger's Textual Commentary on the New Testament does not discuss any significant textual variant here. According to Tischendorff's apparatus, there is a redaction of the Codex Siniaticus that omits και from the phrase και ο ζων, but nothing else.
The oldest complete Patristic commentary on Revelation was written by Andrew of Caesarea (533-637), and has been translated by Dr. Eugenia Constantinou. Andrew here writes:
And when I saw him I fell down at his feet as dead. And he laid his right hand upon me saying, "Do not be afraid. I am the first and the last and the living one. And I became dead, and behold I am living unto the ages of ages. Amen.
Christ revived the Apostle [John] himself who had died through the weakness of human nature like Joshua son of Nun (Joshua 5:14) and Daniel (Daniel 8:17, 10:9-12) by saying to him, "Do not fear, for I have not come near to kill you, since I am beginningless and endless, having become dead for your sakes."
Oecumenius of Isauria, who slightly predates Andrew, wrote:
The holy John would not have been strong enough to survive his astonishment had the saving right hand of the Son of God not touched him, which by the mere touch had accomplished so many wonderful things. And he said to me, “I am the first and the last,” which is as though he had said, “I am he who for the salvation of you all sojourned among you in the flesh at the end of times, even though I am the First and the firstborn of all creation. How is it possible that anything evil transpire from my appearance? For if I who am living and am the wellspring of life became dead for you, and trampled death underfoot and lived again, how is it possible that you who are living become dead on account of me and my appearance? And if ‘I have the keys of death and of hades,’ so that I make dead and make alive those whom I wish, and that I will bring down to hades and bring up again, as it is written concerning me, and that, as the prophet says, escape from death belongs to me (Psalm 68:20), I would not have sent my own worshipers and disciples to an untimely death.”
One other Church Father who comments on Revelation 1:18 is Cyprian of Carthage (210-258). In his Latin translation of the verse in Ad Quirinium, II.26, he also reads ο ζων here.
Eastern Orthodox writers are usually particularly sensitive to being faithful to patristic interpretations of Scripture and highlighting differences of opinion between the Fathers. Lawrence Farley's commentary here explains:
He tells John, “Do not be afraid,” and in this He tells all of John’s churches not to fear. They need not fear death, martyrdom, or anything in all the world. Why? Because Christ has overcome the world, trampling down death by death. He became dead, but now He is alive to ages of ages. As such, He is the first and the last, sovereign over all (compare God as the Alpha and Omega in 1:8) and the Living One, the source of all life. He had authority over death and Hadesby His Resurrection. Death cannot now separate us from Him, for He is Lord of both the living and the dead.1
Russian Orthdox Archbishop Averky Taushev (1906-1976) also wrote a Patristic commentary on Revelation. Here he comments:
From these words, St John had to understand that the One Who appeared was none other than the Lord Jesus Christ, and that his appearance could not be fatal for the Apostle, but on the contrary, would be life giving. To have the keys to something signified among the Jews to receive authority over something. Thus, “the keys of hell and of death” signify authority over the death of the body and the soul.2
Thus, I do not believe there is any credible Patristic source that ever read anything other than ο ζων in the text.
1. The Apocalypse of Saint John: A Revelation of Love and Power (Orthodox Bible Study Companion Series) (Conciliar Press)
2. The Epistles and Apocalypse (Commentary on the Holy Scriptures of the New Testament) (Holy Trinity Monastery)