OBR rules question about fielder positioning

[QuinnVT]

I can't find in it OBR, but in NFHS, it explicitly states that if a fielder is touching any part of fair territory, he is in fair territory . I am asking this in terms of illegal pitches and 1B straddling the bag to hold runner. Had a coach say 1B foot was in foul terrritory , so P balked. We were playing under NFHS rules so I said he had 1 foot in so he was ok. Coach said that wasn't the OBR rule, but he understood we weren't playing under those rules. OBR doesn't seem to tell me - or more likely I coudn't find it, where if a fielder has 1 foot in foul territory, then if the P throws a pitch, its an illegal pitch/balk.