I spent many years retraining/rehabbing OTTbs and here are some thing I learned along the way.
1. I always turn an OTTb out to pasture for 6 MONTHS before restarting. This is because you never know WHAT the trainers have pumped into these poor horses systems and also it helps to have a 'let down' period for the horse to just be a horse, lose the intense level of fitness and grow mentally and physically - remember these horses are start when they are just 18 months old.
2. During this time I make sure I get them nutritionally balanced with top quality minerals/vitamins and SALT (Vital if they are on green grass). You would be amazed at how many of these horses come in with major nutritional deficits that make them behave in strange ways most people attritbute to 'just being a OTTb'.
3. I check their mouths for any damage - these days I ride all my horses in a Dr Cooks bitless bridle which they go beautifully in or if you prefer a bit use a MYLER snaffle. These bits are the best and kindest bits I have ever seen particularly for these horses who very often have very bruised roofs of their mouths due to hard hands in a plain snaffle.
4. Always remember that Tbs are the most SENSITIVE horses. Be gentle and CALM at all times.
5. If they aren't doing what you want them to do it is more than likely that they just don't understand what it is you want.
It's wonderful you are going to give one of these fabulous, loyal and intelligent animals a home. Love him/her to bits and they'll return it 10 fold.
Hope this helps