Oak Lake sits in the south-western corner of Manitoba in the Westman region, within the Rural Municipality of Sifton, at an elevation of 426 metres above sea level. It covers approximately 39 square kilometres (about 6,796 acres) with roughly 21.68 miles of shoreline, making it the largest lake in the region. The lake is a kettle lake, not a man-made impoundment, though an earthen dam at the southern end (Oak Lake Dam) is used to regulate water levels.

The primary surface inflow is Pipestone Creek, which originates in Saskatchewan and enters the lake at the north-west corner through Oak Lake Marsh. That marsh is notably four times the size of the lake itself. Approximately 90% of the shoreline is swamp and marshland, which limits accessible shoreline but creates a highly productive, nutrient-rich aquatic environment. This extensive wetland fringe is the dominant landscape feature paddlers will encounter around most of the lake's perimeter.

A documented access point is located on the north side of the lake at Cherry Point, which offers parking. The Oak Island Resort and Campground, situated on the lake, operates a marina and provides facilities including a campground with over 400 sites. The resort is open seasonally, roughly late June through early September.

Water quality is a consideration for paddlers: an algae advisory is in place, and the lake is not considered suitable for swimming. Paddlers should be aware of potential algae blooms, particularly during warmer months. The lake is noted as a popular destination for water skiing, boating, and sailing, so paddlers should expect motorized boat traffic, especially on weekends during the summer season.

The surrounding area includes the small community of Oak Lake approximately 15 kilometres from the lake, as well as nearby towns such as Virden, Souris, and Brandon, all of which can serve as supply points. The region's history as a stopping point for fur traders and settlers in the late 19th century adds some cultural context to the landscape.