Billionaire investor Warren Buffetts company Berkshire Hathaway (BRKa) disclosed some changes in its holdings in a regulatory filing Tuesday after the bell, including big cuts in its Big Oil holdings. Berkshire slashed its stake in Exxon Mobil (XOM) by 67 percent in the fourth quarter, according to the filing, and cut its stake in ConocoPhillips (COP) by 34 percent.