Continental Resources Inc Chief Executive Harold Hamm has decided not to attend an OPEC event this week in Vienna, according to a company spokeswoman. Hamm, the founder of one of the largest U.S. shale oil producers, had been listed for weeks on OPEC International Seminar’s website as a speaker. Hamm’s attendance had been widely anticipated as he had derided the Organization of the Petroleum Exporting Countries as a “toothless tiger” in 2014. Hamm’s withdrawal leaves only two U.S. shale executives confirmed to speak at the event, out of an original five. Reporting by Ernest Scheyder