Scikitlearn grid search random forest using oob as metric?
Check out the make_scorer function in sklearn: http://scikit-learn.org/stable/modules/generated/sklearn.metrics.make_scorer.html

You may need to code the OoB component yourself, I haven't ever used that metric for scoring before with Sklearn, so I don't know for sure.

Also, consider using random search rather than grid search, it is more practical when dealing with a large hyper parameter space, since it does not need to search the entire dimension of a parameters that have no impact.