Where is the oil cooler located? Does it have a water jacket?

Hi,it might of got water from the water jacket when takeing off the head for rebiuld,I once had a toyota crown and I done a rebiuld and had the same thing,I found out that in the front of the head was a core-plug which actuly was behind the timing gear wich just went strait into the bottom of the sump,well I hope yours is the first diagnoses,all the best.

if it is an automatic ( make and model omitted in question) there is every chance that it is transmission oil from a burst cooler in the radiator tank. The fix is a new radiator or and air to air cooler core fitted in front of the radiator. If the engine is a diesel or an engine that has a sump oil cooler then the oil core has burst allowing engine oil into the radiator vial the coolant system. It may be from a failing head gasket where the oil hole to the rocker gear is leaking across to a water jacket

Oil is entering water system via a small leak in the head gasket adjacent to the oil drilling where the oil comes from the pump through the head gasket up to the head itself to lubricate the camshaft / valve gear. Or, there is a problem with the oil cooler found on a lot of Renaults. If you look at the oil filter there may be a metal donut looking piece with 2 x 3/4 water hoses attached to it. Because the oil pressure is 40+ lbs and the water is 14lbs when running, and if the cooler has an internal leak / fracture, the oil will gradually pass to the water jacket by virtue of the greater pressure.
Disconnect the 2 hoses and join the 2 block ends together with a length of hose, refill the coolant, run the engine for a while and see if there is any trace of oil or bubbles coming from the now open oil cooler stubs. If there is then the cooler is leaking internally. Replacement is the only option. The biggest problem you face is that the engine block, radiator, heater matrix are all carrying the oil sludge that's been circulating the water system. Cleaning all that out successfully may be your biggest quest.
How do I know all this, I'm just doing the same job on a Renault Clio diesel!!
Get some quality help, prepare yourself for changing the head gasket, timing belt etc. I'm going to flush mine with a strong detergent mix on reassembly, running a garden hose into the upper rad stub and allowing the lower to vent out and hope it brings most with it. Only a hot washer would help and I've bought a new header tank from online to replace the discoloured one when there's no gunk left. Good luck.

If you have an auto box then the transmission oil cooler is leaking into the radiator and the fix for that is a new radiator.. Have a compression test done to check for the gasket . If the oil is black then you may have a warped head and engine oil is passing the gasket to a water jacket hole.

check for oil cooler or raditor assy or water jacket...

its either a oil cooler if fitted and if no oil cooler then its a head gasket ,the seal for the oil feed to the head has failed and the oil pressure is pushing into the water jacket via the head gasket .Sorry no quick fixya solution here just hard dirty work

Ii doubt it, most cars do not have oil coolers as thev oil is kept at a moderate temperature by the water jacket of the engine Some high end cars Cadallac. Mercedes Benz etc. may have dual core radiaters where the oil is circulated. See: http://www.ehow.com/about_5087654_oil-cooler-benefits.html

Only two ways for that to happen. Either you have a blown head gasket (gasket de-laminates and permits oil to travel to a water jacket) or your automatic transmission cooler (actually inside one tank of the radiator) has failed. Unless someone accidentally poured oil in there, that's the only way oil can enter. Reddish oil is from the transmission, brown is engine. After repair, make sure you change/replace the oil in which ever system is turns out to be as coolant can also enter through the same place, especially after you turn it off and coolant pressure is higher than oil pressures..

this sounds like head gasket failure to me the seal on the head gasket tends tp break between the oil and water jackets