bleh... p0420 codes. finding the source is a pain in the tuchas.
i wouldn't go to the expense of summarily changing out the O2 sensor(s) just yet. that's not to say a wonky O2 sensor isn't the culprit. (if you have a set of known-good OEM O2 sensors handy, go ahead and swap the rear one first, clear the ECU, and evaluate--if the code returns, replace the front, clear, and evaluate--if the code returns, the source is elsewhere.) same goes for the cat. you
definitely don't want to replace that hurty-expensive part unnecessarily.
to be completely blunt, p0420 codes are troublesome for the shade-tree mechanic to t'shoot. there are nearly a dozen things that can cause that code, none of them pleasant. i humbly suggest you schedule some quality time for your Accord with a local shop that you trust, preferably one that is Honda-centric (Google it), and purchase a diagnostic.
proper t'shooting typically requires use of a professional diagnostic scanner to capture live engine data during back-pressure and vacuum tests. explain to the shop that you just want the diagnostic--not any remedial work. if the report comes back as "faulty front O2 sensor", then you can most likely source and install a new OEM sensor yourself. if you get really lucky, maybe it'll just be a leaky compression gasket at the exhaust manifold.
if you wanna try t-shooting the code yourself, you may find
this thread a useful resource.