The O'Shannassy Reservoir is an Australian man-made water supply dammed reservoir. The 3.123 GL (687 million imp gal; 825 million US gal) water store across the O'Shannassy River is located near the locality of McMahons Creek, approximately 80 kilometres (50 mi) east of Melbourne, Victoria. The dam that creates the impoundment is called the O'Shannassy Dam.

The reservoir is part of Melbourne's water supply system. Water flows under gravity to Silvan Reservoir, then to storage and distribution reservoirs around Melbourne. It is the smallest of the water storage reservoirs managed by Melbourne Water, with a capacity of approximately 3.123 GL (687 million imp gal; 825 million US gal), but it is on a very productive catchment, with stream flow averaging 80 GL (18 billion imp gal; 21 billion US gal) per annum.

The location was selected as it is at sufficient altitude for gravity supply to the elevated eastern suburbs of Melbourne. A diversion weir on the O'Shannassy River and aqueduct to the Surrey Hills Reservoir in Melbourne were completed in 1914. The weir was complemented by the construction of the dam and its reservoir in 1928, but the weir was still used to divert river flows into the aqueduct. The construction of the Yarra-Silvan conduits in the 1950s resulted in reduced requirements for the aqueduct which was decommissioned in 1997.

• List of reservoirs and dams in Victoria
• Melbourne Water