An SEC filing Wednesday shows it paid a weighted average of $54.41 per share, a total of $985 million for the new shares.
At Wednesday's close of $52.99, the Occidental shares are worth $959 million.
In total, Berkshire now reports owning 136.4 million common shares of the oil giant, currently valued at $7.2 billion. That represents more than 13% of OXY's outstanding shares.
Berkshire also holds warrants to buy nearly 84 million more shares at an exercise price just under $60 per share. They were obtained as part of the company's 2019 deal that helped finance Occidental's purchase of Anadarko.
When they are included, Berkshire's stake is almost 22%, and that's the number the SEC pays attention to.
All of the 136 million common shares now held by Berkshire were purchased over the last 13 trading days.
Shares of Occidental have soared more than 80% this year as oil prices hit multiyear highs. Berkshire Hathaway's Class A shares closed above $500,000 for the first time ever on Wednesday.
On March 7, CNBC's Becky Quick reported on "Squawk Box" that Buffett told her, "We started buying on Monday [Feb. 28] and we bought all we could." Since then, Berkshire has added 45 million more shares to the 91 million shares purchased during the first week of buying.
"I read every word, and said this is exactly what I would be doing. She's [CEO Vicki Hollub] running the company the right way," Buffett told Becky.
Berkshire's purchases quickly put its stake, including the warrants, above the 10% trigger that requires it to disclose additional purchases within two business days.