I had also hear that OGE Hover power was defined as the power required to ascend vertically at 200fpm while in an OGE hover...(This claim seems a little far fetched to me...)
Hover OGE is defined as that condition where an increase in height above the ground will not require an increase in power to hover.
MG is right to direct you to ACs 27 and 29, where you will find a wealth of information. Of particular note - for your question - is the fact that the preferred method of data acquisition is by tethered hover and a load cell. Free flight is not the optimal technique for a number of reasons - highlighted in the ACs!