I`ve have had two OTTBs in my life so far -- loved them both. They key to our mutual happiness was full time pasture and minimal oats. When I needed to put weight on one of them, I used soaked beet pulp and vegetable oil (sometimes canola, sometimes corn) but still kept the oats to a minimum.
I think the other posters have offered some good insights and suggestions so no need for me to add the same. She`s a smart looking mare -- good luck with her.