Onana scored in the 2-1 win on Saturday.
He later said: "Unbelievable feeling. We spoke about it yesterday and I was like 'watch me I'm going to score today'. I think if you manifest it, it kind of works so very happy with that today.
"West Ham are a tough team to play but I think we could never have done it any better. Starting with three points away gives a massive confidence to the whole team so I'm happy tonight.
"I think we didn't lose our focus and I think you saw it in the first minutes of the second half. We kept doing what we did in the first half and we did fantastically well so I'm just happy with the overall team performance."
On playing under Unai Emery, he added: "For sure. I think he played a massive part in me coming to this football club. I think his football IQ, his knowledge, what he can bring to me is just phenomenal. I think I can learn a lot playing under him."