They are not the same thing.
The ONVIF specifications provide a fairly standardised interface at the command and control level to allow discovery of devices, determine what capabilities they support, and a means of configuring and using those capabilities. In amongst that will be providing the parameters needed to set up an RTSP stream.
RTSP is a video streaming protocol.
So to answer your question - RTSP will be common to both ways of gaining access to a video stream.