Berkshire now owns 264 million Occidental shares worth about $12 billion, a 28% stake in the big U.S.-focused energy company.

Shares of Occidental Petroleum Corp. shed 4.64% to $46.65 Friday, on what proved to be an all-around poor trading session for the stock market, with the S&P 500 Index falling 0.50% to 6,040.53 and Dow Jones Industrial Average falling 0.

Occidental Petroleum's fundamentals and valuation have improved, presenting a long-term buying opportunity amid oil and gas market volatility. Learn more on OXY stock here.