Go easy on the oats for now. I'd give half for now fed twice daily until the hay puts some weight on her. Getting weight on her needs to be a slow process as too much feed taxes the system. When I'd bo't a horse, not quite this thin, he was started on one cup of oats 2 x daily for 5 days then one cup for another 5 days. He needed deworming but he also needed to put on a little weight first. Your vet should assess this mare to determine if she has sharp teeth, worms by fecal test, etc. I wouldn't look for a richer feed at this time, as the liver and kidneys need to adapt to the change as well as the gut. If you can get a water kefir culture you can grow your own probiotics for her at a cost of 2 T. Of sugar per batch. The culture breaks down the sugar in 2 or 3 days which, after you strain the culture (looks like opaque crystals) to add to the next batch of sugar water. Give her the water as it's ready. The culture will grow so any time you have extras they are good for you to eat (tasteless) or add to the horse's feed. Lots of into on the internet.