How useful would the space shuttle OMS actually be in atmosphere?
Not all that useful - the OMS engines had 6,000 lbf of thrust each.  For comparison, the Allison J35 (used in the F-84) produced 5,600 lbf of thrust.

Comparing weights, the shuttle weighted 165,000 pounds empty (reference) while the F-84G weighed 11,470 pounds empty.  Since we have two OMS engines on the shuttle, we're double that to 22,940 pounds. So we have to push approximately 8 times the weight.

There are a few other considerations - The OMS engine only had enough fuel for 1,000 ft/sec of delta-V, fuel would be needed for entering orbit, and later 200 ft/sec for reentry, along with whatever else was used for maneuvering during the mission. Therefore there probably wasn't a whole lot of fuel remaining. Also in my comparison don't forget that the shuttle is not known for being very aerodynamic, so you might be able to manage small changes, but nothing of significance.