May 1 (Reuters) – Occidental Petroleum said on Friday CEO Vicki Hollub, one of the most prominent women in the male-dominated oil industry, would retire next month, handing the reins to insider Richard Jackson. Reuters had reported in March that Hollub was preparing to retire after a decade at the helm of the U.S. oil and gas company. Jackson, who joined the shale producer in 2003 and is currently its chief operating officer, will join the …