A method and medical device for determining a cardiac event that includes determining classification factors in response to a sensed cardiac signal, and classifying the cardiac signal as one of a plurality of cardiac events in response to the determined classification factors. The classification factors include determining sensed event windows having predetermined interval pairs during a predetermined time period, determining whether a number of the determined sense event windows is less than an interval pair threshold, determining whether a number of intervals occurring during the predetermined time period and not within the sensed event windows that are less than a predetermined interval threshold is greater than a sensed interval threshold, determining whether a predetermined number of intervals occurring during the predetermined time period and not within the sensed event windows are less than a short interval count threshold, determining whether a predetermined number of intervals occurring during the predetermined time period that are either within the sensed event windows or not within the sensed event windows that are paced events is greater than a paced event threshold, and determining whether oversensing detection occurred during the predetermined event time period.