Oscar was always a pleasure to see, both when he would get annoyed (or I should say GROUCHY) at things and when he would delight in something (like seeing 2 people argue) and do his vintage "heh heh heh" laugh. One time I tuned into SS around the late 80's or early 90's I saw a very neat clip of Siskel and Ebert talking with Oscar outside his trashcan. Oscar asked them what they do when they don't like a movie and they showed their famous thumbs down. Ebert gave an example of a movie he didn't like but Siskel liked it. So the two started arguing like they frequently did on their show back then. And Oscar is just delighting in the whole thing. (And I think the movie they were debating was a fictitioous movie, because Ebert mentioned Prince Charming and someone else as the star actors in the movie. He criticized their performances and Siskel says (in a hilarious annoyed tone), "I thought they were both terrific."
But my fave Oscar moment of all time has to be the skit with Grover where they take turns saying the letters of the alphabet and a word that goes with it. And Oscar's words are all negative (eg. D is for Disgusting!, S is for Sickening!). Very funny skit. Oscar is a great character (as is Grover, as is Cookie Monster, as is Ernie, as is Bert, as is Big Bird, as is Kermit, as is....)