p 当然是 p 音 不可能发 b 音

I think when "opposite" is spoken quickly, it comes out closer to "obbisite", which might be giving you confusion when hearing it spoken quickly. To be more specific, I think the "p" is not being aspirated when spoken quickly. There is probably regional variation as well. As others have said, stick with 'p' pronunciation, for more standardized English.

its pronounced with the p

Use the "p" sound. I'm trying to think of any case where a double-p is pronounced anything other than "p". No luck.

It is /p/.