2001 Subaru Outback what's going on with my car?

My check engine light was on and I took it to the dealer, they told me my catalytic converter needed to be replaced. So I took it to Midas and while I waited for the part to get in, driving my car as little as possible, I noticed that it started to have acceleration issues. When going around a corner, down shift to 2nd and accelerate out of the turn, my car would cough and chug like it would if it were low on gas, but it wasn't. Some thing would happen when going from being stopped at a stop light and going up a hill. The only way I could get it to run fine was to keep the clutch in and rev the engine and slowly accelerate.
My first thought was that it was the bad catalytic converter, but I got it changed, and the problems persisted. So then I thought the fuel filter might be bad because I had that problem with my last car, but then I did some reading and saw that it might be a bad o2 sensor according to other people's symptoms of their cars. I asked the mechanic at Midas if a bad o2 sensor would make my car run like that and he assured me that there is no way a bad o2 sensor would make my car chug like that. I think he was full of it. Does someone have some help for me? Is it possibly the o2 sensor and could it make my car run like that?