I think he's just a darn good liar, using his past achievements and boyish looks ,and wearing a suit that's a bit big for him to appeal to the Judge's good nature,and repetitive references to his faith and god etc.....she even mentions about going to Church on Sundays. He even said after he was sworn in at the start of his Cross Exam with Nel that he would 'try to tell the truth' ,,,,,,,who the heck says that if they intended to tell the truth ? very very odd,and can't think for a minute that the Judge and Assessors didn't view that as a red flag from the outset.......very very odd. They liked Oscar, they had a paralympic medal winning Olympic Champion infront of them crying a wretching and saying he'd made a mistake, prays a lot and begging to be believed - they fell for it hook line and sinker. The Judge ignored or twisted all the damning evidence against Oscar as if some of it didn't exist - you couldn't have made it up.
Personally , I think that Judge was far too inexperienced to cope with such a high profile case.