"This enraged [someone, usually 'his father'], who punished him severely." ExplanationIn reference to Hitler's father repeatedly punishing Hitler severely. Kept being used after the Hitler episodes for any form of punishment.
"Dude. Uncool." ExplanationA common in-universe Dude, Not Funny! reaction.
"But OverSimplified, [Person B] didn't [do the thing], the Vikings did." ExplanationOriginally in reference to whether or not Christopher Columbus discovered America. Eventually used to refer to things the Vikings obviously didn't do.
"There's a tax for that." ExplanationA joke from the American Revolution Part 1 video on how the British Empire taxed the dickens out of the Thirteen Colonies.
"More economic downturn." ExplanationIn the Emu War video, the Great Depression is explained through a series of events that create a chain reaction, resulting in — that's right, more economic downturn.
"TO THE GUILLOTINE!" ExplanationMaximilien Robespierre's Character Catchphrase in the French Revolution videos.
Is cereal a soup? ExplanationThis debate started in the American Civil War video, and has appeared in several other episodes since.
"I can't. My gout is acting up." ExplanationIn the Henry VIII video, the Pope's representative overseeing Henry's marriage annulment/divorce trial, Cardinal Campeggio, repeatedly citing his gout acting up for him dragging on the trial.
"NOOOOOOOOOOOOO!!!" ExplanationCommon as a form of comical overreacting.
"OoOohNoOooOooo!" ExplanationA common Oh, Crap! reaction.
"What a wonderful time to be alive!" ExplanationPresent in several episode openings while introducing a historical setting or time period, followed by another person reminding the speaker about how it sucks to live in that time period.
"Hollywood should've hired Oversimplified instead." ExplanationAfter Ridley Scott's Napoleon was released to a very mediocre-at-best reception, people began to note that Oversimplified's videos on the Napoleonic Wars — which are about an hour in length in total — were both more accurate and entertaining than Ridley Scott's film, which is roughly two-and-a-half hours long. The jokes were written in minutes.
