Best settings for fill flash , OM-D?
With a shot like this, it can be difficult to balance the main subject, the interior of the room and the outside.
The outside exposure is defined by ISO, shutter speed and aperature.
The interior exposure is defined by ISO, aperature and flash power. Shutter speed is less relevant other than to prevent double exposures between ambient light and the flash in the event the subject moves.
You need to balance the two exposures to get the effect you are looking for.
I think you started in the right vein by metering the outside scene, though I think you (correctly) nailed the land rather than the sky. I would set the shutter speed to around 1/100 to isolate any movement of the main subject - unlikely in this case but I mainly photograph my toddler who doesn't sit still for long. I would then manually set the flash and fire off a test exposure. You can then tweak the flash power depending on whether your main subject was under/over exposed. This sounds a little fussy, but with a little practice, this becomes pretty simple and quick.
If you want to balance the main subject and the rest of the room, a bounce/swivel flash makes this achievable if the room isn't too large. With a non-bounce/swivel head, you will get differences between the main subject and the rest of the room given the fall off in light as the distance from the flash increases.
I'll post an image where I think I got this right, give or take.