a constant rolling in a futures curve... being as it that oil is in contango right now.. every roll is selling low and buying higher causing a negative drift.. people call it different things.. read the prospectus..
$USO closed up 1.15 percent on Tuesday, August 18, 2015, on 76 percent of normal volume. The bears made the stock sink to a new 52-week low. Due to the stock's strong downtrend, its oversold condition may remain that way and should be discounted.