There is nothing wrong with saying "oh wait". It is simply one of those words or phrases commonly used as an expression. In this case, it can be used to signal that you were too quick to make a judgement or statement, and therefore, made an error. Thus, by saying "oh wait", you are basically saying, "before you make a judgement or fully comprehend what I just said, let me correct myself." Alternatively, it can be used somewhat sarcastically (or not) to emphasise the absurdity of unnecessary details or accuracy - for example, "she claims I owe her five thousand dollars. Oh wait - four thousand nine hundred and sixty-three dollars."