I realize that you are just quoting that 34% from the recent 13G/a, but it isn't exactly correct. OXY has a weird way of disclosing average share count because they have outstanding warrants and depending on the share price some are considered anti-dilutive or counted on sometime of hybrid/cashless exercise method where they would be less dillutive.

So today Berkshire owns (that we know of) 243,715,804 common shares of OXY vs. their last disclosed share count of 880,370,837. That would be 27.68% of OXY common shares currently. (not 34%)

But Berkshire also owns warrants to purchase another 83.86 million OXY shares $59.62/share. It hasn't exercised these and probably won't until right before they expire many years into the future.

OXY also has 101.4 million $22-strike warrants outstanding that have not yet been exercised.

If Berkshire and the rest of the warrant holders all executed their warrants in full there would be approximately 1,065,629,686 OXY shares outstanding and Berkshire would then own 30.74% of OXY. (not 34%)