Who is the Alpha and Omega mentioned in Revelation 1v8?
In context, the one speaking in Revelation 1:8 is Jesus Christ. He is the one ‘coming with clouds’ who ‘was and is and is to come’. This cannot be said of God. The Lord Jesus Christ is the beginning and the end of the purpose of God in that none can come to the Father but by him. None can attain to the kingdom of God, but by virtue of his sacrifice - his life of perfect obedience even to suffering death on the cross and his resurrection to immortal glory.
Revelation 1:4 John, to the seven churches which are in Asia: Grace to you and peace from Him who is and who was and who is to come, and from the seven Spirits who are before His throne, 5 and from Jesus Christ, the faithful witness, the firstborn from the dead, and the ruler over the kings of the earth. To Him who loved us and washed us from our sins in His own blood, 6 and has made us kings and priests to His God and Father, to Him be glory and dominion forever and ever. Amen. 7 Behold, He is coming with clouds, and every eye will see Him, even they who pierced Him. And all the tribes of the earth will mourn because of Him. Even so, Amen. 8 "I am the Alpha and the Omega, the Beginning and the End," says the Lord, "who is and who was and who is to come, the Almighty."
In Revelation 1:11 the phrase occurs again. And again it is Jesus speaking – the one like the son of man – the ‘one who lives, and was dead, and is alive forevermore; and has the keys to the grave.
Revelation 1:10-17 I was in the Spirit on the Lord’s Day, and I heard behind me a loud voice, as of a trumpet, 11 saying, "I am the Alpha and the Omega, the First and the Last," and, "What you see, write in a book and send it to the seven churches which are in Asia: to Ephesus, to Smyrna, to Pergamos, to Thyatira, to Sardis, to Philadelphia, and to Laodicea." 12 Then I turned to see the voice that spoke with me. And having turned I saw seven golden lampstands, 13 and in the midst of the seven lampstands One like the Son of Man, clothed with a garment down to the feet and girded about the chest with a golden band. 14 His head and hair were white like wool, as white as snow, and His eyes like a flame of fire; 15 His feet were like fine brass, as if refined in a furnace, and His voice as the sound of many waters; 16 He had in His right hand seven stars, out of His mouth went a sharp two-edged sword, and His countenance was like the sun shining in its strength. 17 And when I saw Him, I fell at His feet as dead. But He laid His right hand on me, saying to me, "Do not be afraid; I am the First and the Last. 18 "I am He who lives, and was dead, and behold, I am alive forevermore. Amen. And I have the keys of Hades and of Death.
The phrase occurs again in Revelation 21:6, but in context it seems that it is God himself on the throne who is speaking:
Revelation 20:11,12 Then I saw a great white throne and Him who sat on it, from whose face the earth and the heaven fled away. And there was found no place for them. 12 And I saw the dead, small and great, standing before God, and books were opened. And another book was opened, which is the Book of Life. And the dead were judged according to their works, by the things which were written in the books.
Revelation 21:5-7 Then He who sat on the throne said, "Behold, I make all things new." And He said to me, "Write, for these words are true and faithful." 6 And He said to me, "It is done! I am the Alpha and the Omega, the Beginning and the End. I will give of the fountain of the water of life freely to him who thirsts. 7 "He who overcomes shall inherit all things, and I will be his God and he shall be My son.
God says in Isaiah:
Isaiah 41:4 Who has performed and done it, Calling the generations from the beginning? ‘I, the LORD, am the first; And with the last I am He.’
In Revelation 22:13 the phrase occurs a final time and in context it is Jesus speaking.
Revelation 22:12-16 "And behold, I am coming quickly, and My reward is with Me, to give to every one according to his work. 13 "I am the Alpha and the Omega, the Beginning and the End, the First and the Last." 14 Blessed are those who do His commandments, that they may have the right to the tree of life, and may enter through the gates into the city. 15 But outside are dogs and sorcerers and sexually immoral and murderers and idolaters, and whoever loves and practices a lie. 16 "I, Jesus, have sent My angel to testify to you these things in the churches. I am the Root and the Offspring of David, the Bright and Morning Star."
From this it seems that Alpha and Omega in its ultimate sense refers to God who is the source of all life, but in terms of the salvation of mankind it is also true of Jesus. He was the first to be raised from the dead to eternal life.
I hope this helps.