Do I need new gaskets and bolts to install this?

Yes. The factory gasket set will not seal properly on a new header. Source a quality manifold gasket kit and stainless steel header bolts rated for exhaust heat — do not reuse factory hardware.

Does this header come with a downpipe flange or O2 sensor provisions?

The header includes a 4-1 collector outlet. Verify your downpipe interface matches before purchase — adaptation may be required depending on your exhaust system configuration.

Will this require engine bay modification to fit?

Test-fit required. Headers are tight in the Accord bay. Some steering or suspension components may need clearance verification; minor adjustments are common on tight installations.