Oilgoing into radiator

i resently removed the header to fix astriped threads but a week has gone bye and now my oil is leeking into my radiator but no radiator fluid is inthe of the engen what did i do wrong or what happend and how do i clead my raideator or is it ruind

Automatic transmission or manual?

automatic

Check your transmission fluid. The transmission cooler is built into the radiator, and if the cooler leaks transmission fluid can contaminate the coolant.