Next session my group will be arriving in Triboar, which as far as I am aware is named in honour of St Oswin Boldfist's defeat of three large boars, according to the Terror at Triboar Dungeon Masters Guild adventure. Oswin was apparently canonised by the cult of Gwaeron Windstrom (I didn't even know saints existed in FR!), and honestly I'm not sure why - presumably not for fighting three CR2 (if we generously assume 'large' = 'giant') pigs. Before I retcon the three boars into a boar-headed hydra or something, am I missing some information somewhere that explains why Oswin's deeds were thought to warrant canonisation, given how rare this appears to be in FR? Official sources only, please.
We do not
Every source I have found on Triboar refers to the exploit that named it as being a "tale" of a "wandering traveler" and makes no attribution to any saint.
In point of fact, I was (at the time of posting this answer) unable to locate any source that mentions the name "Oswin Boldfist" at all, and I had done a fairly thorough search across every edition of the game. I'm very curious whether you found this reference in the DM's Guild adventure "Terror at Triboar."
We could conjecture...
These elements are fact:
Gwaeron Windstrom was deified by Mielikki for his battles with the avatar and servants of Malar.
Gwaeron Windstrom is said to sleep in a stand of trees just west of Triboar.
Malar is god of marauding beasts and monsters.
Mythology has used boars as terrible marauding monsters before (Calydonian Boar, Erymanthian Boar etc.)
So we can speculate some hypothetical origin story, like
Perhaps at some point during his time as a mortal (or afterwards, why not), Gwaeron Windstrom was weakened from battle with Malar's forces and needed to rest. Oswin Boldfist discovered a trio of dire boars, empowered and enraged by Malar's touch, hunting for the weakened Windstrom in order to kill him. Boldfist heroically pursued the boars, leaping onto one of them and turning its goring tusks into the flank of a second. The smell of blood caused the third to whip around in frenzy, and Boldfist stood fast against its charge. When Windstrom woke, he found the daring mortal lying atop the three slain boars and recognized the great service that had been done him.
But given the tone of the adventure...
The adventure Terror at Triboar suggests that "St. Oswin" is more of a local hero than an actual saint, the founder of a small town with a curious tale for a bit of color. Patrons at the tavern mutter about how they're tired of hearing the story of Oswin, which - as you've noted - is a bit sparse on relevance and heroics. It would be just as plausible, working from this source, that Oswin was never actually sainted and was given the title as a courtesy of sorts in recognition of his local status.
City of Splendors: Waterdeep, Forgotten Realms Campaign Guide, Forgotten Realms Campaign Setting, In Volo's Wake, Powers and Pantheons, Princes of the Apocalypse, Storm King's Thunder, Sword Coast Adventurer's Guide, The North: Guide to the Savage Frontier, Terror at Triboar, The Savage Frontier, Volo's Guide to the North, Waterdeep and the North