Wall Street's biggest banks have changed sides and are embracing a surge in oil prices.

Goldman Sachs Group Inc., Morgan Stanley and JPMorgan Chase & Co. have all issued a flurry of bullish forecasts in the past fortnight. They've abandoned their skepticism and are accepting that OPEC's output cuts are finally succeeding in clearing a global glut.

This faith is a recent phenomenon. At almost every stage of OPEC's quest to end the industry's worst downturn in decades, there were a few prominent analysts ready to cast doubt on the effort.