You have several options for this case. You can either make approximations and make a guess on the torsion terms, develop your own parameters, or we can develop the parameters for you and guide you through it (but this will take us about 6 weeks to get to).
So if you'd like to make some approximations, you could set the Os-Cg-Os-C to the same term as a Cg-Cg-Os-C, assuming that if this were a hexane, its torsion term would be similar. And you can set the H2-Cg-Os-C to 0 so it won't have a torsion term.
Let us know if which plan works best for you.