Thank you for your question. I do not have a lot of information about the Oneness Pentecostals. However, I think they are related to the group that calls themselves Apostolic. According to the "Handbook of Denominations in the United States: 11th Edition," there is a branch of the Pentecostal churches called the "Church of our Lord Jesus Christ of the Apostolic Faith." They have about 30,000 members in 450 churches. They are "non-trinitarian, maintaining belief 'in the oneness of God, who was the Father in creation, the Son in redemption, and today, He is the Holy Ghost in The Church.' " They also believe that the baptism of the Holy Ghost (presumably with tongues) is necessary for salvation. This denomination was organized in 1919 in Columbas, Ohio, by Robert C. Lawson who became the "chief apostle" of the wider organization.
This teaching is so bizarre that I am not sure where to begin in refutation. However, it is clear that they are the ones who believe in three different gods: one for creation, one for incarnation, and one for speaking in tongues. To deny that these three operate at the same time is amazing. It is easier to understand someone who denies the deity of Jesus Christ and the Holy Spirit. But, according to this teaching, Jesus Christ (who is "the same yesterday, and to day, and for ever" Hebrews 13:8) used to be the Father and then became the Holy Ghost. This is infinitely removed from scripture.
Then, to claim that the baptism of the Holy Ghost (with the tongues and such that they identify with it) is necessary for salvation; well, it is just too much. Clear Bible teaching means nothing to them. The Apostolic preacher that I used to hear on the radio here in Knoxville just kept going to the same few verses over and over again. And these he twisted to say what he wanted them to say. Their salvation is not God's salvation as revealed in the Bible. May the Lord help you as you witness to this man who is probably lost himself. For anyone who is trusting in such an experience for salvation is probably not saved. "Believe on the Lord Jesus Christ, and thou shalt be saved" (Acts 16:31).