The fact that the leading and trailing edges don't have any rounding off or ringing suggests to me that it's not the scope, however it could still be a slew rate limitation in the scope, or it might be your oscillator.
If you can lay your hands on another scope, try connecting both to the same calibration output and see if the trace looks the same, or perhaps changes when one of the scopes is connected. (the other scope should have its own calibration output, so you could compare that too.