I have 2 ottbs and the both didn't like snaffles try like the friend link type ones on a snaffle with a lougens, so it doesn't have the nut cracker affect
Patience and lots of second chances!!!
Definitely make sure you leave all your stresses and problems at home. They read off body language better than most breeds I have realized. They are smart and catch on quickly so keep it interesting so they don't get bored. BUT! Don't confuse stubbornness with actual cluelessness, so be very clear on what you ask from them. They are a fantastic breed!! You will learn so much from them!! Have fun!
Be very calm and patient. Be very meaningful with your aids, so they know exactly what you mean. Use very light hands and a soft bit. Talk to your trainer and have lessons frequently. You may need a standing martingale to start off just for safety as some OTTB's like to throw their head in the air.
Always stay calm around them, because they're usually energetic calm energy is key