Ottbs arent really completely broken in. They are taught to go really fast in a big circle and eventually stop when they are ready lol
In the past I've approached retraining an ottb like I were dealing with a young, green horse and spend a lot of time on groundwork. Lunging is an important place to teach him what he needs to know under saddle. Plus the added advantage of lunging time is you can smooth out imperfections from the relative safety of the ground rather than in the saddle.
An old ottb I had years ago struggled with the concept of flexing around corners etc and was constantly dropping his shoulder and running through the bends. Was very annoying :) so I started leading him around in the arena doing the same patterns I would if I were riding and whenever we came to a bend I would tap him on the shoulder with the crop to teach him to bend around it. He was pretty responsive on the ground the moving away from pressure etc but he didnt get it under saddle at first :) it took me some time but eventually he got the idea and figured out what I was asking him to do. The transition to doing it in the saddle was really good and although we still had a ways to go he had improved heaps. Once we got this he also started to figure out things like slowing down and listening more to me. Each thing he learns make him that little bit more mature :)
Spend a lot of time just being friends with him too. Thoroughbreds are very loyal horses and are so eager to please. If you build a great friendship with him by doing things that he likes he will be even more eager to please :) and I believe that to truly be effective when retraining an ottb you need to have a relationship with the horse.
Go slow, learn about him and have fun with him :) play your cards right and you will have a friend for life