DC3:

A couple questions:
1. What size battery are you using, and do you know how much it weighs?
2. Did you get your engine mounted as far forward as possible, or is it shoved back against the base of the engine mount?
3. How far back from the LE of the top wing is it balancing?

Mine weighs 10 pounds 3 oz, and was initially balanced at 3.4" aft of the top wing LE, which is 25% of MAC and is where this airplane should balance for its first flights, given that the tail section is so tiny. I used the following items:

-A magnum XLS 91 two stroke engine (24.8 oz w/stock muffler) mounted as far as I could possibly get it out on the end of the engine mounts, to where the fake radial engine would not fit and I made a plywood panel to hide the stuff and insure proper engine cooling. As you can see, the carb inlet actually pokes through my panel a little, that's how far forward I shoved that puppy. I have a monster prop on it, too- a 15x5 Zinger wood.

-A 2200mAh 4 cell square pack (8.4oz), mounted to the bottom of the engine mount in a plywood box I made for it. I used long engine mount bolts that go through the bottom of the box, and then used some nuts to secure the box to them. You can see in the pic that it's also right up behind my plywood panel.

-The one lb block of lead that comes with the kit. I also moved the throttle servo forward of all the others, but this was for ease of running the pushrod, not balance so much.

-A brass Higley ultra heavy hub holding the prop on, weighed about 4 oz. I have since removed and replaced it with an aluminum version of the same thing, as I wanted to move the CG aft just a tad, to help with nosing over in strong(er) winds or high(er) grass than I'm used to. Haven't flown it yet with this mod.

Your OS 61FX weighs 23.6 oz w/muffler, almost as much as my 91, so I'm guessing moving it way out on the mounts, combined with possibly a bigger battery, might do the trick for you and it will weigh about what mine does. It's worth it to buy another mount and relocate it, if that's what the difficulty turns out to be, and to get a bigger battery, as that just gives you that much more flight time.

Hope this helps. Be assured you are doing the right thing- being careful with initial balance. More new birds are lost to tail heaviness than anything I can think of. Don't hesitate to do whatever it takes. This bird has lots of wing and can handle the weight.