It's difficult to answer from your brief description but it could be a blown head gasket, bad radiator, stuck thermostat, or the cooling system may be air locked. The oxygen sensor could be causing it to run rich.
Before digging into a head gasket do a few more things....
Does the Cooling fan(s) come on? No, check relays? If it has 2 fans turn A/C on, one should come on.
Check engine oil, is it over full and milky looking? Indicates Blown Head Gasket
Open Coolant over flow bottle is it bubbling if so does it smell like exhaust fumes? Indicates Blown Head Gasket
Answers & Comments
It's difficult to answer from your brief description but it could be a blown head gasket, bad radiator, stuck thermostat, or the cooling system may be air locked. The oxygen sensor could be causing it to run rich.
Before digging into a head gasket do a few more things....
Does the Cooling fan(s) come on? No, check relays? If it has 2 fans turn A/C on, one should come on.
Check engine oil, is it over full and milky looking? Indicates Blown Head Gasket
Open Coolant over flow bottle is it bubbling if so does it smell like exhaust fumes? Indicates Blown Head Gasket