Are you certain that the MAF sensor wiring harness is hooked up? Mine did this same thing when I unplugged the connection to get more slack in the tube when installing a new filter. Could the plug be what you think is an O2 sensor?
I also ran an oiled K&N with no CEL, and now a Hot Shot tube with an oiled cone filter w/o a CEL. I'm betting your connection is the problem. If the harness is connected to the side of the MAF, disconnect it and check the condition of the connectors, then try plugging it back in.
"If you're not living on the edge --- you're taking up too much space!!"