oem is the best if you will keep your car longer.

anyways, magnaflow could work also as long as it is not a high flow cat then you might still have a problem.

im a smog tech so here are some stuff that are related in controlling your nox emmisions

egr----not working (if your car is eqquipped with one)

overheating problem

a part of your cat that controls nox is not working anymore.