I reccomend purina strategy foal grow formula... I've posted about it before but I really lovvveee this stuff and so does my filly. She only gets about 2.5lbs a day but she's been growin' like a weed in all directions it seems like... It's also pretty safe, you could if you were crazy feed it as a hay supplement so it's pretty fool proof in my opinion. I add biotin and flax oil to it a couple times per week for healthy hoofs and coat.
Not to mention it's pretty inexpensive, approx 20$/ bag
Why are you feeding oats & what else are you feeding? A basic low starch, high fibre diet of free choice(or little & often) hay/grass and appropriate nutritional supplements to give her what she needs should about do. I'd consult a nutritionist or use something like feedxl.com to ensure a well balanced diet. A proportion of the diet could be lucerne/alfalfa or such, which is high in protein, energy & calcium can be good but may be too much for 'easy keepers'. Health problems(DODs for eg) can arise if you try to 'grow up' a foal too quickly or allow them to become overweight.