Ok, there'* alot of things in play here. Your air is entering the front of the box on one side. It then goes through a square filter, and exits the airbox to the intake plenum on the other side. That'* restriction, but it'* flowing better than stock. The biggest problem is that the K&N filter will get dirty right behind where the air comes in. Rotation of the filter (180° flip) would be a good idea come springtime. As the filter gets dirty there, it has to go to a different part of the filter to find a way through. Path of least resistance.
Sometimes it can take up to 100 miles for your PCM to re-learn after a major change. In your case, with a suspect O2, your 'learning' symptoms could have been worse. The PCM is adapting to changed inputs from your MAF and IAT right now. Just give it time. If you don't get at LEAST 26 highway, change that O2.
Your fuel curves are all 'up in the air' right now as far as the PCM is concerned. To be safe, disconnect your battery for 30 minutes, re-connect it, and go through an entire tank of gas (without changing octane), then check for performance/efficiency.