The Oneness Pentecostal people, as a whole, absolutely believe they have the truth and that the Trinity doctrine is pagan in origin. With this in mind, when you speak to them, you'll encounter an attitude that they are absolutely right. It is very difficult to break through this and often takes a lot of time and effort. Think about it. If you are very convinced of your position in contradiction to theirs, how would you feel if they tried to convince you you were wrong? It'd be very difficult to do.
Nevertheless, it is important to keep a comfortable and humble dialogue going with them. Pride is not the answer to error. God's love must come through. So, when witnessing to them, or anyone, you need to show love and respect. They need to know the same love of Jesus in you that they claim they have from God. Jesus said that the world will know that we are His disciples by the love we have for one another (John 13:35). Of course, being loving means being truthful with them. If you don't know something, admit it. If you aren't sure about something, that's okay. Be truthful.
You also need to demonstrate to them the love of Jesus in your life. You need to emphasize that Jesus Christ is the center of your faith. Oneness Pentecostals believe that they are the only ones who have the true love of God. In that sense, they believe that they are a privileged group of people enjoying a special knowledge and special relationship with God. That is why they emphasize so much the need to follow their doctrines and be baptized in their churches by their ministers and according to their understanding of the gospel. They honestly believe it. So, you need to be patient, kind, and truthful with them in your witness.
Also, and this is quite common, correct any misunderstanding of the doctrine of the Trinity. So often, opposing groups have a misguided concept of the Trinity and end up attacking a strawman argument. Of course, to correct any misunderstanding, you must first understand it yourself. So, if you're rusty, read up on the Trinity. Then, when you think you've got it down, you can share what you know with them. Show Scriptures that demonstrate the Trinitarian nature of God.
You will need to demonstrate some of the problems with the Oneness Pentecostal position--and they exist. In order to do that, you will need to read more about them and the issues here on this website as well as other websites and books on the subject. Unfortunately, witnessing to those in opposing theological beliefs often means that you have to study theology. It isn't that tough. It just takes some work and practice. But it is always worth it.
Do you know what they believe? You cannot refute error if you do not understand what you are talking about with them. Personally, I like dialoguing with them on the internet to learn what they believe. Reading Oneness material is helpful, but it doesn't answer all the questions that I have. Bouncing things off of them gives me a feel for how they think as well as what they think. That helps me witness to them a lot.
Finally, pray. You must ask the Lord to bless your efforts and bring fruition to the seeds planted. It is God who bears the fruit of truth.
"So then neither the one who plants nor the one who waters is anything, but God who causes the growth," (1 Cor. 3:7).