Yes, a leak in the exhaust manifold upstream of the O2 sensor changes the exhaust/air-ratio, and the O2 sensor will be relaying incorrect information back to the ECC.
When you replaced the sensor, did you reset the ECC? The ECC has to "re-learn" the system when you swap out sensors with new ones, otherwise it will act like the old one is still installed.
You can reset by either disconnecting the battery completely and leaving it sit overnight, or, disconnecting the battery and touching the positive terminal cable to the negative terminal cable. Both ways will drain the capacitors in the system and your ECC will attempt to relearn the system within the first 50 startups.
I had to do this several times when I replaced the TPS, IAC, O2, and CPS to correct an idling problem I had back last Summer.