Indeed - although I suspect it is from learners that you mostly hear it. It has spread as a result of classes, I think. I bet (certainly from my experience at least) that most native speakers would say o'n i'n meddwl rather than ro'n i'n meddwl.
In the imperfect generally, the forms without r- (and indeed likewise without the d- in the NEG) are always OK in natural speech, though the language planners don't like it. I do though.
O'n i'n meddwl - I thought
O't ti'n meddwl? - did you think?
O'n i ddim yn meddwl - I didn't think