If it heated up on you, chances are that you warped the cylinder head and now have a blown gasket. You don't give very much information, here.
Oil/antifreeze mix is usually caused by head gasket leaks or valve cover gasket leaks.
If you didn't get it warm after the work and then refill the tank - the system was probably closed when you filled it. It's possible once the thermostat opened (or just as the coolant leaked past), the fill you gave was used as it filled the system past that point.
Hoses? Intake manifold? Heater? Thermostat housing? Freeze plugs? Radiator? Pressure test system to help find
First thing you should check is the antifreeze level. If that is fine, replace the thermostat. If it still over heats after the thermostat, then most likely you need to replace the waterpump. If your antifreeze is low, check for leaks, the radiator, hoses, heater core, and water pump. If the water pump is leaking, that is your problem, replace that and thermostat all in one.
have you flushed your radiator or changed the thermostat it could be stuck
smell of antifreeze and no visable leaks usually indicates that the heater core needs replacing
let some antifreeze out,remove two bolts holding thermostat cover on,its located on the end of top of radiator hose.remove thermostat ,remember how it goes back in,scrap old gasket off and wipe down,then apply a coat of silicone on thermostat cover and intake and set your gasket on and place your new thermostat on after checking in 180 degree water to see if it opens and closes.put cover on and screw two bolts back on and add antifreeze,crank and check for leaks.
3.1 engine leaks at the intake gasket
A thermostat that is stuck in the closed position is a very common cause of overheating. A new thermostat should be installed every time the coolant is changed.
drain some of your antifreeze and disconnect your top radiator hose from cover,then remove 2 bolts holding cover on,remove thermostat and clean gasket off both surfaces,check new thermostat to see if it opens at 180 degrees,then apply sealent to both surfaces and install gasket and thermostat and 2 bolts and tighten and put hose back on and tighten clamp and add antifreeze,then crank and check for leaks.
Bad head gasket or... Crack in exhaust port in head. Check exhaust for presence of antifreeze smell or exhaust gas in radiator.
Same color as what?