If you are using OpenFX plugin to connect with DaVinci Resolve and the image plays back on it's own in OmniScope even if it's paused in Resolve it means that you have Caching enabled and the background task is processing the frames which are then sent to OmniScope.
There are 2 options to fix the issue:
Disable caching in Resolve,
Use the OpenFX on an adjustment layer.
To prevent OmniScope from playing back frames while Resolve does the caching, simply create an adjustment layer above your timeline in the Edit Page, and place the OpenFX plugin on top of that layer.
Find Effects in the Toolbox in your Edit Page on the left side of your timeline:
And then drag the OmniScope Connect plugin on top of that Adjustment Clip: