From my understanding (although this might have changed), it's slightly different for OU, and parent/s' income doesn't count.
I don't think your dad will be considered as a parent. Your mum's income might need to be taken into consideration, but probably won't as you've been living independently (can't remember what the cut off age is to be a "mature student" for SF.
As well as phoning SF, it might be worth a phone call to OU as well.
Good luck with the course 👍