You never said if you fixed that exhaust leak or not. If the leak was before the O2 sensor than you could be loosing oxygen content out of that hole so the O2 would think there was no need for more fuel and you would run lean.

What are some good places to get O2 sensors? My Dad's CRV has needed one for a long time. OEM Honda is $180, he will spend that but its like pulling teeth to get him to finally buy it.