Translation:Yes, I was
Right, normally when answering in the past tense you'd use do/naddo, however, for the imperfect tense (was), then you answer with the oedd form instead (which in everyday speech is shortened down to o'n).
Unfortunately, Welsh doesn't have a simple word for yes/no like English does, and I'll admit to still getting confused by it myself, don't worry! :)
So, the answer to a question phrased in the perfect aspect would be "do" or "naddo," but the answer to a question phrased in the imperfect would be "o'n" or whatever the negative of that is. Thanks, I was not getting that at all. I think there was a perfect thrown into the batch of imperfects or vice-versa and I got all confused.