I may get in trouble for this but there is a whole sub-forum on octave mandolins, mandola, citterns, bouzouki, mandocello, etc. over on mandolin cafe.
I'll share my experience: I wanted to get into octave mando and bought a Michael Kelly Octave plus. Not a great instrument. I later bought an octave from Tom TJ Jessen that I love and play often. He makes truly great
A and
F style OM's. I've played Webers, Clarks, Mowry's, Herb Taylor - and they are great. But one of TJ's will sound awesome for 1/2 to 1/3 the price. Joe Mendel also builds some great octaves.
Many other people are quite happy with Trinity College octaves as well.