Melt butter in a medium saucepan over medium. Add onion and cook, stirring frequently, until onion is soft but not browned, about 2 min. Add orzo and wine. Stir often until orzo has absorbed all of the liquid, about 1 min. Add broth, 1 1/2 cups water and oregano to pan and bring to a boil over high. Then reduce heat to medium-high. Stir often until orzo has absorbed most of the liquid, 9 to 10 min. Risotto should be slightly saucy but not soupy. Stir in half of the feta until it is melted and risotto is creamy. Stir in remaining feta, olives and parsley. Garnish with grape-tomato wedges.