Ford Repair: 97 ford mustang gt 4.6, blown head gasket, side tubes

Question
what causes the oil to mix with the antifreeze in the radiator and how can i fix it. and i have had it flushed.  

Answer
Hello and thanks for the question.....I'll try to help.
 Are you sure it's oil and not transmission fluid (if it's an automatic)? If it is transmission fluid, I would replace the radiator, because the transmission fluid cooler is in the side tubes of the radiator, and that would be the place of the leak.
If you are positive it's engine oil, look for a blown head gasket, crack in the head(s) or crak in the block.
Hope this helps,
Erik