Registered: Oct 2017
Nah, it's not canon. But elements of it are. Some authors, like Mike Chen (author of Brotherhood) have stated that they see the OCW as holonet propaganda, reporting on a hyperbolic version of real events.
For example, we know Kit Fisto fought alongside the Mon Calamari against Separatist-backed Quarrens on Mon Cala, but the events were drastically different.
We also know Mace Windu faced down an "overwhelming wave of Super Battle Droids on Dantooine," but the actually size of the force has never been confirmed, and there was no mention of some impact-based superweapon in use.
Similarly, Anakin recalls facing one of Dooku's agents on Yavin IV, but we don't know if his duel with Asajj happened exactly as we saw it in OCW.
Lastly, Ki-Adi Mundi had scars from his fight with Grievous when the cyborg general first revealed himself to the Jedi, but again, we don't know that it played out exactly like the battle of Hypori in OCW.
So, I'd say OCW is "Essential Legends," like The Thrawn Trilogy, Darth Bane Trilogy, X-Wing series, ect. Material that, while no longer canon, is definitively helping to shape the current narrative of Star Wars storytelling.