I say,"ooh ooh ooh ooh, ahh ahh ahh ahh", until he learns to say it and then I added to that ,"Hey Monkey!" and now when I say, "Hey Monkey!" he says, "ooh ooh ooh ooh". He has not learned the,"ahh ahh ahh" yet but he will.
My parrot's name is Alex and I've always thought his name perhaps has put too much pressure on him and on us because he is named for a famous parrot of the same species and that Alex can do many tricks and say many words and phrases. When I give our parrot a nickname it feels more fun and relaxed. No burden of proof is on anyone to meet unspoken expectations. And, even though our Alex is very well spoken, he is uptight as most of his species are and anything that relaxes us and him is a good thing.
He seems to enjoy the hollow,echoing sounds of the "ooh", maybe it reminds him of the sounds of the green jungle his ancestors are from and from which he is not too far removed.