By Pete Najarian, co-founder of OptionMonster.NEW YORK ( TheStreet) -- Berkshire Hathaway ( BRK.B) is not a frequent name on OptionMonster's systems, but last week it saw some unusual option activity. Buyers snapped up the September 90 calls in the B shares of Warren Buffett's company for 88 cents to 93 cents on Thursday, according to OptionMonster's tracking systems. More than 5,500 contracts traded, about twice the previous positioning in the strike. Calls lock in the price investors must pay for the stock, so they can provide huge leverage if it goes up. But they can also expire worthless if the shares fall or don't move enough. The stock fell 0.17% to $80.91 on Thursday and is at the upper end of its range in the last year. The company is an investment managing firm that primarily engages in the insurance and reinsurance of property and casualty risks. Calls outnumbered puts by about 9,200 to 1,200, a reflection of the bullish sentiment. Najarian has no positions in BRK.B.