MEXICO CITY — The administration of President Andrés Manuel López Obrador of Mexico has launched its first major anticorruption prosecution, delivering on a central campaign promise by taking aim at corruption inside the ailing state-owned oil company. Bribery and tax fraud charges have been filed against Emilio Lozoya Austin, the former head of the state-run oil company, Petróleos Mexicanos, or Pemex, who was a top ally of the former president, Enrique Peña Nieto, officials said Tuesday. Mr. Lozoya is accused of receiving bribes in connection with the oil company’s purchase of a fertilizer...