Understand oshidashi
The Japan Sumo Association describes oshidashi as a winning technique in which one or both hands are placed against an opponent’s chest or underarm area to push them outside the ring. For a first-time viewer, the useful detail is the pushing contact. Notice the relationship between the hands, the moving bodies and the boundary. You are trying to recognise the technique in a demonstration or match, rather than learn a physical method from a short paragraph.
Understand yorikiri
Yorikiri has a different description: the wrestler advances forward or sideways while maintaining close body contact and forces the opponent outside the ring. That close relationship between the bodies is the key contrast to observe. The two finishes may initially look alike because both end beyond the boundary. Comparing them teaches you to look at what happened before the final step, and gives you a more precise question to ask during a guided introduction.
Make the distinction useful
At a private session, ask whether the instructor can explain the difference with examples appropriate to your programme. Demonstration, observation and visitor participation are separate things, so let the instructor decide what is suitable to practise. Later, use the official technique pages to refresh the vocabulary. There is no need to memorise the entire catalogue before enjoying sumo. Recognising these two ideas already makes a bout more interesting and helps you appreciate the choices behind a finish.