No, not that kind of “O”; or, at least, I don’t imagine so. This looks more like an “Oh shit, what’s he doing?” face, or perhaps an “Omigod, I had no idea it was possible to experience that sensation!”
Perhaps we could blame the weighted nipple clamps, but they’re padded and the weights look to be resting on the floor. However, observe that our intent man in charge has got a power cord running over her knee to whatever electrical appliance he’s deploying between her legs in the vicinity of her nether regions. Since this is not a government photograph, we can assume the device is not a soldering iron. So, what’s he got?
My money’s on a violet wand, or perhaps a powerful vibrator.