Omen Stribog can be found in Omen's Hideout in the River Of Frozen Tears, Polaris.
On the map of River Of Frozen Tears the hideout is located top right - it shows as a little black arched cave entrance.
If you haven't reached there yet you can port in to use your keys but will need someone to go in via the sigil (which takes you into a cave before using the key). You'll be needing a team anyway and at least one of them should be high enough level to access it without porting.
TBH, if you're high enough level to be accumulating gold keys it would be worth questing there so you don't need to port. Polaris is a beautiful world and sadly a little on the short side...but that does mean you can quest to Omen quickly and easily
