I think two of the most important factors for LOR's is the person's enthusiasm for you (they like you, trust you, want you to succeed, etc.) and thoroughness (the go into specific examples for why you would make a good OT). It's definitely helpful for all of them to be somewhat familiar with OT so they can speak to your skills and strengths as they relate to the field. If they aren't, I would talk to them beforehand and let them know what qualities the program is looking for. They could talk about ways in which you work well with others, act in a leadership capacity, stay calm under pressure, use creative problem solving and express yourself well both verbally and in writing. One of my recommenders filled out the rating system in the OTCAS evaluation and then only wrote a few sentences about me. I found out later that this LOR was rated as mediocre by my program. I was only three points away from getting into the program, so I really kicked myself for that weak recommendation. A lot of programs rely on LOR's to get a sense of who you are before they meet you, so you really want people who are excited about you to write them.
