It's a distorted version of "O puta naman."
"Puta" is a Spanish loanword which means bitch or whore. "Naman" is a Tagalog enclitic particle that is used to show contrasts, softens requests, and emphasise things. It doesn't have direct English translation. In this case, "naman" emphasised "puta" so "O puta naman" is just another way of saying "Oh bitch!"