BlackCherry83

My engine light is on and the code suggests its the o2 sensor. I reset the light and it passed smog. Tightened gas cap at suggestion from my dad but light comes back on.
My engine runs a bit rough to me.
I replaced the timing belt and broken water pump. Better but still a bit rough.
My car has 105k miles.
A friend has suggested
- fuel injection, but from what I've been reading its a rip off?
- replace catalytic conveter; again, not sure if necessary?

Is this likely just the o2 senser? Any other ideas?
What other maintenance might I look into besides regular oil change? I want my car to last a long time.