Oil tanker
Oil tanker = wanker ('wanker' is a British English term whose dictionary meaning is a person who masturbates, but generally just implies a low opinion of that person.)
Oily rag
Oily rag = fag (cigarette- in British English, 'fag' isn't used as a a slang term for a gay man)
On your knees
On your knees = peas
Orange peel
Orange peel = feel
Otis Redding
Otis Redding = wedding
Otis Redding was a singer.
