On the Oneness and the Multiplicity of God
All prophets, Jewish, including Jesus ﷺ as well as Muhammad ﷺ have confirmed that God is One.
God is also omnipresent; there is nowhere without God. The matter, the laws of nature, all things and all living beings are from God, and manifestations of Him. Hence the manifestations of Him are many, uncountable even, but they are not God.
As your words, your deeds and your thoughts are not your being, but manifestations of your being, the Word of God, conveyed by the Holy Spirit to the Prophets to whom we listen, or implanted in ourselves without our knowledge, are not God, but a manifestation of God.
In the history of Christianity, there have been some scholars, called «monophysites», saying that Jesus is God, others denied. The Orthodox Church (including the Roman Catholic) has invented a compromise between those opinions, saying that God, Jesus and the Holy Spirit are both, one and distinct.
The message of the Quran clearly states that neither the Holy Spirit is God nor Jesus is God, the first implicitly, for the latter, see Quran 5:72-73
They disbelieve those who say, “God is the Messiah the son of Mary.”
But the Messiah himself said, “O Children of Israel, worship God, my
Lord and your Lord.” Whoever associates others with God, God has
forbidden him Paradise, and his dwelling is the Fire. The wrongdoers
have no saviours.
And indeed, Jesus himself clearly denied that he is God, as in Mark 10:8
And as he was setting out on his journey, a man ran up and knelt
before him, and asked him, "Holy Teacher, what must I do to inherit
eternal life?" And Jesus said to him, "Why do you call me holy? No one
is holy but God alone.
or in Mark 13:30-33
Truly, I say to you, this generation will not pass away before all
these things take place. Heaven and earth will pass away, but my words
will not pass away. But of that day or that hour no one knows, not
even the angels in heaven, nor the son, but only the Father. Take
heed, watch; for you do not know when the time will come.
… and indeed, it came out that Jesus did not know it because he was wrong in his first sentence.
Formally, one could say on this base, the compromise is both, false and true. But, also formally, it is not possible that something is both, false and true. Indeed, a compromise between what is true and false is false.
On the Oneness of a human with God
There is something completely different, called Oneness with God. The concept, probably coming from Islamic Sufi tradition, taken over by Judaism and Christianity, is to try to approach a spiritual state where the own will is in full accordance with the Will of God. The Christian medieval Scholar Meister Eckard called this (ideal) state of spirit «Oneness». This Oneness with God is however not a kind of being-the-same-as God but a property or a state of the soul. An extraordinary grade in reaching this state is certainly in particular necessary for a prophet but not exclusive to prophets. This is meant when Jesus ﷺ uses the term «Son of God» for humans – for himself but also to his disciples and followers.
The Quran avoids this term because its message is directed to people who were converting from their polytheistic religion, but also to Christians who falsely mixed up the original meaning of the term with the Greek pagan concept of humanoid deities having intercourse with humans making «half-gods», from where it arose that Christians prayed to Jesus or even to Mary (calling her «mother of G-d») which is clearly corrected in the Quran (4:171)
O People of the Scripture! Do not exaggerate in your religion, and do
not say about God except the truth. The Messiah, Jesus, the son of
Mary, is the Messenger of God, and His Word that He conveyed to Mary,
and a Spirit from Him. So believe in God and His messengers, and do
not say, “Three.” Refrain—it is better for you. God is only one God.
Glory be to Him—that He should have a son. To Him belongs everything
in the heavens and the earth, and God is a sufficient Protector.
The aim to become one with God in its proper sense is good, the misconceptions are not.
The link between the Oneness with God and the Onenss of God
Now, how should we approach the state of Oneness with God if we do not understand God Himself as One?
Oneness with God means to follow the Will of Him. How could we follow His will if we see Him divided into aspects? No, we can't.
We may study the manifestations of His Will but we will not succeed until we bring them together as a whole.
Oneness with God means to respect His creation. We have to see that all is from and in His Hands. It is not holy, as God alone is Holy, but a manifestation and a grace of Him, and all belongs together as all originates from the Same and goes back to Him.