Do you know how many fillers/spin-offs/added material exist for One Piece? After 20+ years of popularity, obviously a lot. It can be hard to distinguish them sometimes, especially because there are some gray areas depending on Oda's involvment.
I'll try to shed some light in this brief chapter.
Movies, light novels, (video)games, OVAs, live shows, specials, cross-overs and anime fillers are generally non-canon. [I hope every anime watcher knows which ones are the filler arcs/episodes but in case you're confused, hit me up - or I can make a list here.]
BUT unfortunately it's not that easy, as there are exceptions and mixed moments.
Obviously, anything that happens in the manga is considered canon, together with the cover stories and the info in the SBS.
For extension, the scenes adapted in the anime are also canon. But don't forget that there are filler episodes and also filler scenes often included in canon sequences - those are all non-canon. They do that to stretch out the stories so the anime doesn't get too close to the weekly serialization of the manga, the same reason why the pacing of the anime sucks sometimes.
There is one exception though (because it would be almost too easy otherwise): episode 50 "Usopp vs Daddy The Father! Showdown at High Noon!", which takes place during Loguetown and is filler, can actually be considered canon as it's the adaptation of an idea Oda had and couldn't fit in the actual manga. So, in his head, it should have happened.
Also, the anime did change some important details of Zeff and Sanji's backstory as it was considered too crude for children.
Ace's two-volume light novel, which tells Ace's story from the start of his jurney to the moment he officially joins WB's crew, is considered canon since it was directly supervised by Oda. He also included his own sketches of all the Spade Pirates. You can buy the books on Amazon, I have them. Note that the first part of the novel was adapted into a manga chapter by Boichi - although he changed or skipped over quite a few details, like the whole Isuka's ordeal.
Interestingly, Law also has a light novel covering his backstory from the moment he leaves Corazon to the moment he forms the Heart Pirates and starts his jurney. So far though it's considered non-canon since it didn't come from Oda's material, but I honestly hope Oda makes it legit in the future (since it includes a lot of info on Shachi and Penguin's backstory and even a Sulong moment for Bepo!). You can read the plot on the wiki, the book is only available in Japanese for now.
As for the movies, despite all the stories being non-canon, they did include some canon material from time to time, first of all Shiki. The "Strong World" volume that was distributed during the airing of the movie in Japan tells the canon story of how he escaped Impel Down. Douglas Bullet and Z are gray-area characters as they were directly designed by Oda, but it's unclear whether they fit in the continuity. Also, Aokiji's canon post-timeskip appearance is first revealed in a movie.
Information coming from the OP Magazine and the characters' sheets should be considered canon, technically, but Oda reserves the right to change or overlook them. So I guess... canon unless Oda decides otherwise? XD Because of this, for example, it's still unclear whether Reiju has a DF or not.
Overall, don't forget that even the official Viz translation sometimes fails (they're still spelling it "Zolo", they re-adapt all the nicknames, they add puns, they leave out the onorifics and talking quirks, etc.) - the original Japanese will always be the real source, although of course we have to take what we can.
I think I covered most things. If you have questions or doubts, feel free to ask! :D
