Heat oil in a medium saucepan over medium heat. Add onion and garlic; cook, stirring, until starting to soften, about 2 minutes. Add orzo and stir to coat. Stir in broth, buttermilk, pepper and salt; bring to a boil. Reduce heat to maintain a simmer and cook, partially covered and stirring occasionally, until the orzo is tender and creamy, about 10 minutes. Remove from heat and stir in Parmesan. Sprinkle with parsley before serving, if desired.