Warren Buffett’s appetite for Occidental Petroleum Corp.’s stock keeps growing, with the billionaire’s Berkshire Hathaway Corp. boosting its stake in the oil-and-gas company by more than $300 million in the past week. Berkshire Hathaway is by far Occidental’s largest shareholder, as Buffett’s investment vehicle now owns 27.7% of Occidental’s outstanding shares. The second-biggest is investment firm Dodge & Cox at 10.3%, according to FactSet data. Occidental’s stock OXY, +2.36% surged 2% in morning trading, ...