Definitely reset the ecu, did you change the exhaust manifold gasket and donut gasket? Don't have any epic exhaust leaks right? What do you mean wiring for the O2, all you should have had to do is screw it in and plug it in. You didn't drop the O2 sensor or anything right? If resetting doesn't work and not exhaust leak try to borrow an O2 or find one at a junkyard and pocket it since you never know if those things work. All else fails get a cheapo ebay status straight pipe and throw it on so you can see if your cats clogged, if it is you can resell straight pipe after you save up for another cat.
I'm willing to bet its either ecu, exhaust leak, or O2 though if you didn't touch anything else
