The button for OverSample will become checked for timebase settings of 20ms or less when triggering on channel A or channel B. Oversampling is disabled for Auto trigger.
When oversampling is active, the display will only show the trace for the channel that is being used as the trigger source. Oversampling can be turned off by clicking the OverSample control.
The timebase setting changes the sampling rate of the oscilloscope. The sampling rate is displayed on the bottom right of the oscilloscope panel shown below.