Key TakeawaysWarren Buffett's Berkshire Hathaway has boosted its stake in Occidental Petroleum to almost 29%.Berkshire bought Occidental Petroleum shares every trading day from June 5 to June 17, adding a total of 7.3 million shares.Buffett's firm now has 255.3 million shares of Occidental Petroleum. Shares of Occidental Petroleum (OXY) edged higher Tuesday as Warren Buffett’s Berkshire Hathaway (BRK.A, BRK.B) bought up even more of the stock, boosting its stake in the energy firm to almost 29%. Berkshire H ...