The logo for Occidental Petroleum is displayed on a screen on the floor at the NYSE in New York.
Reuters bought another 5.99 million shares of Occidental Petroleum Corp and boosted its stake to 20.9% after the company's shares lost a fifth of their value in less than a month.
The purchases were made between Sept. 26 and Sept. 28 and cost about $352 million, Berkshire said in a regulatory filing on Wednesday.
Berkshire now owns about 194.4 million Occidental shares worth approximately $11.9 billion, based on the Wednesday closing price of $61.41.
Berkshire had gone seven weeks since its last purchase of Occidental shares, which pushed its stake above 20%.
The Houston-based oil company's share price went up on August 29, peaking at $77.11 before declining amid broader weakness in the U.S. equity markets. Berkshire paid from high 50 s to low 60 s per share in this week's purchases.
Occidental's share price has doubled this year, benefiting from rising oil prices after Russia invaded Ukraine on February 24th.
Berkshire's stake would allow Buffett's Omaha, Nebraska-based conglomerate to report its proportionate share of Occidental's earnings with its own operating results, through the so-called equity method of accounting.
In August, a U.S. energy regulator gave Berkshire permission to buy up to 50% of Occidental's common stock.
Berkshire also owns $10 billion of Occidental preferred stock, which generates $800 million in annual dividends.