Opal is in western Fauquier County, 7 miles (11 km) south of Warrenton, the county seat, and 18 miles (29 km) northeast of Culpeper. US Route 17 diverges from Route 15 and 29 at Opal, leading southeast 31 miles (50 km) to Fredericksburg. Bealeton is directly south of Opal.
According to the U.S. Census Bureau, the Opal CDP has a total area of 3.2 square miles (8.2 km2), of which 0.03 square miles (0.07 km2), or 0.81%, is water.[2] The north and east sides of the community are part of the Potomac River watershed via Licking Run, Cedar Run, and the Occoquan River, while the west and south sides are part of the Rappahannock River watershed, via Tinpot Run and Bowens Run.