2000 ranger w/3.0 engine. check engine light staying on, could it be an 02 sensor?

 

Could be many things.

You can take it to most parts stores and they'll usually run a scan for free. Better yet, pick up a bluetooth OBD2 scanner. I have and it works great. It tells you everything you need to know, including what error code is causing the CEL to stay on.

They are cheaper than a full handheld unit, but I have never used a handheld one so I can't say how they compare feature wise.

 

+1 ^^^

Get the code(s), there are a few hundred possible codes, guessing is a waste of time and money.
Also Codes are not specific, do NOT replace parts based on a code number, post it here for advice.