OTC exchange can mean anything. Nowadays there are plenty of regular crypto exchanges offering OTC services to their wealthier clients.
As long as there is a centralized entity between you and the person you are transacting with, there is no such a thing as being guaranteed any level of privacy.
I think you are referring to a decentralized exchange? If so, then that's a decent option, but not the best as there is little serious liquidity available which generally leads to massive spreads. I doubt we'll see that change any time soon as people have been begging for decentralized exchanges for years now and there has not been much progress in the liquidity field at all.