"We are speaking about you."
Translation:Mówimy o was.
As soon as you see a preposition, you need to forget about objects etc. and just remember what case is used with this preposition. "O" meaning "about" mostly requires the locative case.
With "o" I recommend to remember the verb, too, as certain verbs will want to have the accusative after "o", like pytać o, prosić o, dbać o.