Warren Buffett added even more to his bet on energy stocks in recent days, buying more than 9 million additional shares of Occidental Petroleum . According to securities filing on Wednesday night, Buffett's Berkshire Hathaway bought another 9.55 million shares of Occidental over the past week at a cost of roughly $529 million. Berkshire now owns more than 152 million total shares of Occidental, according to the filing. The stock was already the seventh-largest position in the company's portfolio at the end of the first quarter . Occidental's stock is up 92% year to date, while Berkshire's Class A shares are down about 10%. Occidental isn't Buffett's only major energy bet. Chevron was a top five holding for Berkshire at the end of the first quarter. Previously, Berkshire scooped up about 14% of Occidental in a two-week period in March. Buffett discussed that trade during Berkshire's annual meeting in April, saying that volatility in the market earlier this year created a " gambling parlor ." Shares of Occidental rose 3% on Thursday after the additional purchases were disclosed.