"Do you have a cat or a dog?"
Translation:Tu ai o pisică sau un câine?
I'm not sure if this is right or not, but my husband says that including the 'o' or 'un' focuses the attention on the number of things - Do you have (a) cat - or two cats, or three? Whereas excluding it is more binary - Do you have (a) cat - or not? With the indefinite article, you're discussing how many; without, you're discussing presence or absence. It seems to be more of a subtle connotation than a hard and fast rule, though.