If the milk is there & just hasn't come down yet the oxytocin will bring it down, but if the mare simply doesn't have a lot of milk, the oxytocin won't make her make more milk. So, if after a few days you discover that the foal isn't getting enough, the oxytocin isn't likely to be of benefit at that point.
Anyway, do watch the foal carefully. The mare may have a small bag & still have enough milk, but don't assume that such is the case. If the foal is nursing a lot, and often, it may be a sign that there isn't enough milk & the foal is hungry. She may be getting enough to survive, but not enough to thrive on--as long as she is lively and doing well/looking good, she probably has enough. If she isn't real active and/or looks kind of rough, not even rough, just not growing/filling out as a foal does in the first week, that is a sign that she's not getting enough milk.