British and Irish Lion Roberts, capped 74 times by Wales, was the first active international player to play in the match since Australia's Dan Vickerman captained Cambridge in 2009.
But the 29-year-old centre was forced off at half-time with an injury that could prevent him from making his debut for new club Harlequins in their Challenge Cup game with Calvisano in nine days' time.
With Roberts watching on from the sidelines, an ice-pack strapped to his right thigh, fly-half George Cullen scored four penalties to steer Oxford to a record sixth successive victory.
Roberts is studying for a two-year masters degree in medical science at Cambridge, qualifying him to compete in the Varsity match, which pits together Britain's oldest and most prestigious universities and dates back to 1871.
Cambridge have won 61 matches, but Oxford's latest success took them to 59 wins. There have been 14 draws.