If mother is o positive and baby is a or b postive, how many will test postive for direct coombs at birth? my son did but didn't have jaundice.

Not automatic: If there are minor blood type incompatibilities, like ABO,not every doctor will test every baby with a direct Coombs. Some only test if the baby shows signs of early jaundice. So it's very difficult to say the exact percentage who are positive because not every baby who fits the question is even tested, let alone keeping track of how many are positive. Being + doesn't equal jaundice as you've seen