Meanwhile, heat oil in large skillet. Add mushrooms; cover and cook 2 minutes.
Add orzo and cook another 2 minutes, stirring, until pasta is lightly toasted.
Begin ladling hot liquid into pasta mixture, about a cup at a time. Stirring frequently, allow each ladleful to absorb before adding another. When all liquid has been used, the mixture will be thick and creamy.
Remove from heat and stir in grated Parmesan. Serve topped with crushed black pepper, if desired.