Warren Buffett’s Berkshire Hathaway has the green light from regulators to purchase as much as 50% of Occidental Petroleum Corp.’s shares, after seeking approval for such a transaction in July.

In a regulatory filing Friday, the Federal Energy Regulatory Commission said that Berkshire Hathaway had asked for and received its permission to buy up to 50% of the driller’s shares. Berkshire has been loading up on Occidental’s shares this year, amassing roughly 20% of the company’s stock, public filings show, leaving many analysts to speculate whether Mr. Buffett would seek control of the company, one of the largest U.S. oil producers.