The purchases were made between Sept. 26 and Sept. 28 and cost about $352 million, Berkshire said in a regulatory filing on Wednesday.
Following the purchases, Berkshire now owns about 194.4 million Occidental shares worth approximately $11.9 billion, based on Occidental's 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 later rose, peaking at $77.11 on Aug. 29, before declining amid broader weakness in U.S. equity markets. Berkshire paid in the high-50s to low-60s per share in this week's purchases.
Occidental's share price has more than doubled this year, benefiting from rising oil prices following Russia's Feb. 24 invasion of Ukraine.
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 of annual dividends.
(Reporting by Jonathan Stempel in New York and Akriti Sharma in Bengaluru; Editing by Subhranshu Sahu)
By Jonathan Stempel and Akriti Sharma