DAVID O’REILLY has made a big bet–big not only by Las Vegas standards, but by the standards of the swashbuckling oil industry. The Chevron CEO bet more than $18 billion that oil supplies will be tight and prices will stay high when he bought Unocal–after the Chinese authorities yielded to political pressure and withdrew CNOOC’s higher, competing bid. He is betting that the fabulously successful boss of Exxon Mobil, Lee Raymond, is wrong to be optimistic about the future growth of world oil supplies. In O’Reilly’s corner are some experts who say that we have discovered just about all of the big fields that exist; in Raymond’s corner is the International Energy Agency, which reminds us that demand might slow and prices drop.