How many oz in three eights of a QT?

Answer 1

A quart is 32 fluid ounces. 3/8 of a quart would be 12 fluid ounces.

Fluid ounces, however, are a measure of volume, and you asked for ounces, which is a measure of mass. Depending on temperature, it's roughly the same for water, and for everyday purposes, most watery fluids. Kool-Aid is heavier than water because of the sugar content, for instance, but no so that anyone notices.

If you give your horse a 3/8 of a quart of whole oats, however, those 12 fluid ounces only weighs about 5.16 ounces. (And your horse will still be hungry. Got some hay or good pasture?)