Driven by a risk-taking chief executive, Occidental Petroleum Corp. is trying to swallow a competitor close to its own size, outmaneuvering one of the world’s oil giants in the process. If it succeeds, CEO Vicki Hollub will win accolades and Occidental will ward off growing threats from the giants in one of the world’s biggest oil fields. If not, Occidental could itself be vulnerable to a takeover. Occidental’s…