mirwin,
I used a Magnum 120, not the OS, but I imagine they are about the same weight. Right now, I balance perfectly with the RX and battery on CG. But, I also did some 'lattice work' on the fuselage to lighten up this known-to-be tail heavy plane. You can read about my mods [link=http://www.rcuniverse.com/forum/m_2496934/anchors_2496934/mpage_1/key_skybolt%252Clightening/anchor/tm.htm#2496934]here.[/link] I also made a hatch in the top for fuel tank access, and made the landing gear area accessible. Mine will also be ready to maiden in the next few weeks. Good luck with yours.