The third and final season began on July 7, 2019. It was confirmed to be the series' last by Ian JQ on August 6, 2019, with the finale aired on September 6, 2019.
The series follows the titular character, K.O., and his efforts to become the world's greatest hero, while working at Gar's Bodega (run by the no-nonsense Mr. Gar), a hero supply shop in the Lakewood Plaza strip mall. Alongside him are his best friends and co-workers Radicles, an apathetic, selfish alien, and Enid, a levelheaded big sister-like employee, as well as other heroes who work and frequent the area.
- Radicles (Rad)
- Mr. Gar
- A Real Magic Skeleton
- Mr. Logic: Lord Boxman's first robot.
- Ms. Mummy
- Red Action
- Bell Beefer
- Mega Football Baby
- Nick Army
- Joff the Shaolin Monk
- Punching Judy
- Ted the Viking and Foxy
- Shy Ninja
- Rippy Roo
- Joe Cuppa
- Doctor Greyman
- Pricipal Claus
- Crinkly Wrinkly
- Dynamite Watkins
- Professor Venomous/Laser Blast/Shadow Figure: the main antagonist of series second half and O.K.'s dad.
- Lord Boxman: the main antagonist of series first half and secondary antagonist of series second half.
- Darrell:was the new owner of Boxmore until Venomous buy back.
- Boxman Jr.
- Billiam Milliam
- Big Bull Demon
- Small Calf Demon
- Chameleon Sr.
- Chameleon Jr.
- Wally the Withe
- Crudde and Spanky
- Miss Quantum/Bad Apple
- Chip Damage
Main article: Lakewood Plaza Turbo
The first six episodes of the first season premiered on the CN app on June 13, 2017 ("Let's Be Heroes" through "You're Level 100!" and later made its television debut on August 1, 2017. The season concluded on April 6, 2018 with the half-hour special "You're in Control".
The first eight episodes of the second season premiered on the CN app on May 4, 2018 ("Seasons Change" through "The So-Bad-ical") and made its television debut on May 5, 2018. The season concluded on the CN app on June 7, 2019 with the half-hour special "Dark Plaza". The season finale later made its television debut on June 30, 2019.
The first six episodes of the final season premiered on the CN app on July 5, 2019 ("We Are Heroes" through "The KO Trap") and made its television debut on July 7, 2019. The series concluded on September 6, 2019 with the television debut of "Dendy's Video Channel" and the television premieres of "Let's Fight to the End" and "Thank You For Watching the Show", the latter of which serves as the series finale.
It is the shortest aired season of the series, just being one day shy of two months.
On September 29, 2017, a crossover between OK K.O.! Let's Be Heroes and Ian JQ's original webcomic RPG World titled "A Hero's Fate" aired. When a long-lost hero stumbles into Gar's, KO agrees to help him complete his years long quest.
Captain Planet and the Planeteers
On October 9, 2017, a crossover between OK K.O.! Let's Be Heroes and Captain Planet and the Planeteers titled "The Power is Yours!" aired. When Lord Boxman is trashing the Plaza with pollutants, K.O, Rad, and Enid recruit Captain Planet to help.
On August 23, 2018, the series finale of Mighty Magiswords, titled "Let's Team Up Because We Aren't Bad Friends" was released on the CN app. It featured a minor appearance of OK K.O.! Let's Be Heroes character, Radicles. Prohyas would later make an appearance in "Crossover Nexus".
On October 1, 2018, a Halloween special of OK K.O.! Let's Be Heroes was released on the CN app, it features characters from Ghoul School as Enid's friends. The special is titled "Monster Party".
On October 8, 2018, a special crossover episode featuring K.O. from OK K.O.! Let's Be Heroes, Garnet from Steven Universe, Raven from Teen Titans Go! and Ben from the 2016 Ben 10 reboot aired. The plot involved KO finding himself trapped inside an mysterious place, where he encounters other Cartoon Network Heroes and they band together to escape.
The special featured cameos of various other characters from many other Cartoon Network series, such as: I Am Weasel, Hi Hi Puffy AmiYumi, Chowder, Ed, Edd n Eddy, The Moxy Show, Adventure Time, Regular Show, The Powerpuff Girls, The Amazing World of Gumball, Uncle Grandpa, Secret Mountain Fort Awesome, Johnny Bravo, Whatever Happened to Robot Jones?, Codename: Kids Next Door, Sym-Bionic Titan, Sunday Pants and many more.
Sonic the Hedgehog
On August 4, 2019, a crossover between OK K.O.! Let's Be Heroes and Sonic the Hedgehog titled "Let's Meet Sonic" aired. When world class hero Sonic the Hedgehog visits the plaza, he teams up with K.O. to save the day.
Thank You For Watching the Show
On September 6, 2019, Strike and Garnet, from "Crossover Nexus" make cameos in the series finale, it also features Captain Planet from "The Power is Yours!".
Appearances in other media
K.O. appeared as a video game character in the Craig of the Creek episode, Power Punchers.
K.O., Rad, Enid, Mr. Gar, and Carol appeared as playable characters in an update of Cartoon Network: Match Land, Lakewood Plaza Turbo also appeared as a stage. Background characters from OK K.O.! Let's Be Heroes (Shy Ninja, Nick Army, Red Action, and Real Magic Skeleton) also appeared but as non playable boardwalk customers.
Rad's haircut and Gar's Bodega uniform appeared as wardrobe in Kleptocats: Cartoon Network.
K.O., Rad, Enid, and Mr.Gar appeared in Cartoon Network: Fusion (not to be confused with Cartoon Network Universe: FusionFall). Gar's Bodega and Boxmore appeared as buildings.
K.O. and Enid were playable characters in the mobile game, Cartoon Network: Plasma Pop.
- T.K.O. (February 17th, 2018)
- Just Like Adventure Time, this show has been burnt-off in the network in Sundays until their series finales has aired and got put in a decent timeslot in a weekday.
- Difference is, this show never had a marathon to conclude the show.