If it is low oxygen due to turnover, you can minimize this with an overflow drain that siphons the water off the bottom of the pond. Read the link below. About 3/4 the way through the article is a section called water control structures. There is a diagram showing a siphoning overflow. You basically put a bigger pipe over the drain pipe so that any water that goes to the overflow has to come up from the bottom of the pond.

http://msucares.com/wildfish/fisheries/farmpond/building/construction.html