I have an OTTB. He raced for 5 years. He accepted his sadle with no problem at all. Just make sure the saddle is properly fitted because you don't want a situation where your saddle causes discomfort to your horse. Good luck and congrats! OTTB's are great horses!
They use saddles on the track. They are smaller and lighter, but saddles nonetheless. Ottb's are saddle broke, bridle broke, broke to ride, but they are trained differently. It's not the "breaking" that is the problem with these horses, it is teaching them their default gate is no longer the gallop.
Read books on retraining ottb's, there's a good one by New Vocations on Amazon.