The Organization of the Petroleum Exporting Countries’ logo is pictured ahead of a meeting between… OPEC’s agreement to slash oil output faces its first test Saturday, when it asks producers outside the cartel to make production-cut commitments intended to amplify the effect of the landmark deal. The Organization of the Petroleum Exporting Countries has invited representatives from 14 countries to meet in Vienna on Saturday to discuss joining the cartel after it agreed last week to slash 1.2 million barrels of daily oil production—more than 1% of global output. In all, OPEC wants its non-OPEC counterparts to slash 600,000 barrels a day. Four countries have committed to attend Saturday: Russia, Oman, Bahrain and Azerbaijan. OPEC officials believe Mexico and Kazakhstan are also likely to come. Their participation is crucial, OPEC officials say. Without it, the oil-price gains of the past week would be imperiled, and the OPEC production deal […]