There is a good solution below to keep your Ovation from slipping. You don’t have to stand up, or use a strap or simply not use Ovation the way you want to. Please read on=>
Ovation's kick ass in a lot of ways and they have their own sound. I have a 1977 USA Glen Campbell that had a crescent shaped anti-slip pad on it that was coming off so I just yanked it off the rest of the way. Ovation used to sell them with this feature but they got away from it in the 80's for whatever reason. I then bought a US made 1978 model 1768 Elite, black with gold h/ware. Both are amplified and both have their place in the studio. When I began playing 45 minute sets on Pearl Street in Boulder, CO I noticed my model 1768 would slip, a real drag when playing live sitting down. And come to think of it the Glen Campbell also slipped when sitting after I pulled what was left of the factory anti-slip pad off.
I have other dreadnaughts I can play on Pearl Street but I happen to like the way my 1768 sounds when amplified a bit through my AC/DC, battery powered Roland Micro Cube amp.
So I therefore needed to keep if from slipping, and this is what I came up with:
1) Foam like kitchen cabinet liners. Cut to size and use silicone to adhere it to the bottom edge of the guitar where it makes contact with your thigh. Don't use Epoxy or super glue or similar adhesive because they are difficult to get off of the guitar when the anti-slip pad begins to wear out and you want to replace it with a new one, or when you want to remove it for whatever reason. I've found that a "fat" oval shape is best.
2) Get anti-slip tape with adhesive backing from Home Depot and Lowes. 3M makes it and they call it Safety-Walk. It comes in pieces about 20” x 5” for about $8.00. Ace h/ware also sells something similar but they are smaller pre-cut pieces and they are $12.00. Use MS Word or MS PowerPoint or similar to draw the correct size oval for your Ovation to use as a template. The size will depend on the depth of the bowl, but you want as much contact as possible for best results. Cut the template out after printing it and trace around it on the backing paper. Then use aviation snips (or your wife’s good crafting scissors if you enjoy sleeping on the couch for a week or more) to cut out the oval anti-slip pad. Carefully install it and your Ovation will no longer slip while playing sitting down.
3) Buy a no-slip pad from the grocery store in the gadget aisle. Women use these to help them open cans and such, and they will also keep your Ovation in place nicely. These pads are about 6” square so you will need to cut to size.
Number 2 (above) is my preferred method because of the adhesive backing. This makes it easier to install. I have never tried number 3, but may if I get a third Ovation.
Good luck with it and hope you can now enjoy your Ovation for years to come, even when playing while sitting down.