Thoroughbreds usually have a fine mouth to bit. Small mouthed, usually low palate, not much room for the bit. Sometimes super sensitive and others can be dull to the aids. More times overs, straight off the track they pull against the hand, yet then can change quickly to being sensitive by carrying their head high and resistant to contact.
Here are my recommendations for the early transition days:
Soft flexible bit to allow the horse to soften into the contact. Better for the TB who travels head high yet soft. Also good for the horse who may have mouth splits. Dressage Legal (PC check your state)
Mouthpiece is similar to that of garden tubing. Also recommend for the horse that carries head high, but he perhaps wants to pull. Dressage Legal
Steady head carriage, yet we are wanting to introduce him to a metal bit. The link is a barrel that reduces movement of the bit. Keeping him steady and not confusing the aids. Soft style which is also good for the OTT which may pull through the aids. Dressage Legal
A great transitioning bit, will teach your OTT to come more into the contact. Giving more weight in the rein. Great for pleasure riding/training as Not Dressage Legal.
A sweet iron bit that offers 50% Tongue Relief and 50% Jaw Relief. The single joint has a lock which prevents the cannons of the bit collapsing onto the bars (jaw) of the horse. Offering small amount of tongue relief will have the horse come more into the hand. Dressage Legal