Money managers have been cutting back their bets on oil in the run up to the crucial meeting of the Organization of the Petroleum Exporting Countries in Vienna this week.

The number of bets taken by hedge funds and other big investors on the global oil benchmark Brent—comprising bets on both rising and falling prices—has fallen to just over 320,000, the equivalent of 320 million barrels of oil, its lowest level in nine months. The number of bets on West Texas Intermediate, the U.S. gauge, is at its lowest since the beginning...