It seems monsters and witches don't get along because monsters know boundaries and consent, but witches don't. A witch can alter reality, weather, and shapes. Monsters can identify plant types. Thus if a witch gets it in her head to mess with monsters, the way Alice did, the monsters don't really have a way to fight back.
In the end of "Chocolate Money Badges" when Oscar becomes Counsellor for a day, both Susie and Alice are shown carrying a bench with Oscar on top in typical Too Important to Walk fashion. But one notable absence is Betsy, who unlike her fellow Witches is not shown carrying Oscar around. This is likely due to Betsy being the nicest witch and the one who told Oscar about the Counsellor for a day prize in the first place, which means that Oscar would have no grudge against her while he would obviously have one against Susie (due to being the meanest witch) and Alice (for turning his monster friends in babies that one time).
Betsy's necklace is a Moon which foreshadows her being a werewolf.
Alice thinks Oscar is cute (as mentioned in "Monster Babies") likely because she is an elephant as well, and of course she would consider her own species to be cute.
Maybe the reason Susie is prejudiced against werewolves is due to her being a cat.
In the episode "Ghost the Boy", after Ghost is returned to his family, Susie attempts to banish one of the campers dressed in a sheet, thinking they were him. What exactly did Ghost do that made Susie decide it was a good idea to send her friend's boyfriend to who-knows-where? Either theres more to Ghost than what weve seen, or Susie is crossing a huge line. The fact that Susie avoids talking about him with Betsy makes one wonder what really went down between those two.
At least one monster baby went over a waterfall, and didn't reappear.
Oscar's choice of words in the first episode when regreting his decision to leave is already suspicious in an of itself: "I left too soon. I didn't give it a chance." But consider he left underwater and that Julia Pott's earlier shorts often have a drowning motif...
Speaking of drowning, in the beginning of an episode, a camper runs off to test if they has the ability to breathe underwater after eating magic ice cream...and is not seen the rest of the episode.'
During the sleepover, a considerable part of the guest list is sentient living food. Oscar pops a marshmallow in his mouth and it starts screaming so he takes it out and puts it back with the others which leads to what do the campers eat!?!