Way back in the day (1960s) when I was a handsome young man
, Oneness Pentecostals were not formed as a Pentecostal denomination yet. In New Zealand independent Pentecostal groups who denied the Trinity were called "Jesus Only" or Branhamites, because they followed the teaching of William Branham. In the U.S. these groups increased in size and the Oneness Pentecostals were formed as a Pentecostal denomination. Many modern generations of Oneness folk don't know that their theology stemmed from Branham's teaching. That is why you didn't initially know about him.
Actually Oneness Pentecostalism comes from a much earlier historical heresy, called, as you mentioned, Modalism. The heresy is called Modalistic Monarchism and was found in the Church in the 2nd and 3rd Centuries, according to Wikipedia: "Modalistic Monarchianism is closely related to Sabellianism and Patripassianism, two ancient theologies condemned as heresy in the Great Church and successive state church of the Roman Empire."
Modalism teaches that God is one person manifested in three different modes.
Manifested as the Father
Manifested as the Son
Manifested as the Holy Spirit.
This is contrary to the Nicene Creed that defines God as three persons who share the same essence.
New Zealand "Jesus Only" Pentecostals teach that the only read person is Jesus, and that He manifested Himself as the Father in the Old Testament, and manifests Himself in the Christian church as the Holy Spirit. This theology denies some of the essential elements of the Gospel - that Jesus is seated at the right hand of the majesty on high, while the Holy Spirit dwells in us, and that Jesus is our defence lawyer in heaven advocating to the Father on our behalf; that Jesus is our mediator between us and the Father. For the Modalist, there is no Jesus in heaven, no mediator, and no advocate, and no Father in heaven. Also, for the Modalist, aka the Oneness Pentecostal, praying the Lord's prayer is meaningless, because it starts with "Our Father who is in heaven". If there is no Father in heaven, then who would the Oneness Pentecostal be praying to if he was praying the Lord's prayer? And if Jesus never went into the heavenly holy of holies to offer up His blood to the Father as the one-time sacrifice for our sins, then how could we be saved? It would make Jesus dying on the Cross for our sins meaningless.
The fact is that Oneness Pentecostals preach another gospel that is not what Paul preached, and they worship a god other than the God of the Bible.