There actually have been episodes when Oscar was depressed, and not mean and Grouchy. Like in the season finale from the 1980-1981 Season (Same one that featured Mr. Rogers) when it was very hot outside and everybody was in Hooper's Store to keep cool. Telly told Mr. Hooper Oscar looked depressed, and he suggested everyone go out there and bother him so he will be himself again. Olivia thought it was a crazy idea, but they all went out anyways, and Oscar was back to his Grouchy self again. Another time he got depressed was in 1993 when a Grouch named Prunella came to Sesame Street when she was driving by Oscar's can and her motorhome broke down. Oscar had hopes of Prunella settling on Sesame Street, because he was really starting to like her. At the end of the episode when Prunella gets her motorhome running again and leaves, Oscar sinks down into his trashcan moaning and feeling depressed and lonely. Big Bird and Snuffy give him a polaroid picture they took of a mess they made in Big Bird's room, but it still doesn't cheer him up.
As for a tearjerking moment. I'd have to say it was around 1997 when a Bird Art Collector named Leo Birdelli offered Big Bird 100 Bags of Bird seed for his picture of Mr. Hooper, but Big Bird explains to him that he could never sell it, and tells him all about Mr. Hooper and they show a few flashbacks of the old days, and in one of these Mr. Hooper is seen with a photo Album and he's singing that goes "When I...... was as little as you are...... I worked in my papa's store. I delivered the groceries and brought back the money, and on weekends I did even more...... When I was as little as you are........" Then it shows Big Bird and Leo Birdelli back in Big Bird's nest area and the they are both quiet. Then Leo asks gently "What happened to him" and Big Bird says "Oh, he died..... and it was very very ver-ry hard saying goodbye to him because I really loved him, I'm sorry Mr. Birdelli, but I couldn't trade this picture for even 1 million billion bags of Birdseed" then Big Bird looks at the picture sadly and says "I can't say goodbye to you again Mr. Hooper" Pretty touching for a 90's episode. It also surprized me they did a tribute episode to Mr. Hooper, since at that time it had been 15 years since he died. I guess it was basically an episode for us classic fans.