What it does
Occa Berry only covers Fire: an opponent lands a super-effective Fire move on the holder, it gets halved, and the berry is eaten.
Fire punishes three groups — Steel, Grass, and Bug — and those groups are packed with high-usage utility mons. Steels wall, Grasses set up, Bugs break, and their shared weakness is Fire. A single Flamethrower or Heat Wave (the doubles spread Fire move) on a 4×-weak Steel or Grass usually just KOs. Occa halves it, turning "burned through" into "still standing" so you keep working that turn.
Heat Wave is the one to watch in doubles: it burns both opponents at once, 95 base power, and a Steel and Grass standing together both die. Occa at least keeps that one holder alive.
Same split from Focus Sash as always: Sash needs full HP and dies to chip and spread damage; Occa works from any HP but only covers Fire. If you know a Fire move is coming for you, Occa is the steadier pick.
Who runs it
The carriers are Steels and Grasses for whom a Fire weakness is lethal.
Tinkaton (12%), Steel/Fairy and 4× weak to Fire, is one of Occa's top users — a single Fire move removes it, but with the berry it survives to keep setting hazards or clicking Gigaton Hammer.
Scizor (7%), Steel/Bug, is doubly 4× weak to Fire and fears it more than anything; a Fire move to the face just deletes it. Occa is one of its few ways to eat Fire and live long enough to Bullet Punch for a kill or Swords Dance up.
Corviknight (7%), Steel/Flying, and Aegislash (2%), Steel/Ghost, both run it to survive. On the Grass side, Tsareena (6%), Venusaur (2%), and Whimsicott (2%) take it to buy a turn for setup or control, and Sinistcha (8%), Grass/Ghost, carries Fire as a big weakness too.
How to use it
The rule: hold it for that one Fire move.
With your berry-holding Steel or Grass out and a Fire attacker across from you lining up Flamethrower, Overheat, or spread Heat Wave, eat the halved Fire, stay up, keep the turn. For Scizor, surviving that hit means a priority Bullet Punch the next turn for the kill.
Mind Heat Wave in doubles: if both your mons are Fire-weak and standing together, Occa only saves the holder — the other still burns. Don't stack two Fire weaknesses on the same side when building.
A Fire move like Overheat drops the user's Sp. Atk after use, so once you eat the halved first hit, the follow-up Fire damage is weaker too — Occa saves you and softens the opponent's second swing.
Tips & strategy
The best use is on a doubly-Fire-weak Steel/Bug like Scizor. Its Fire weakness is public knowledge, so opponents click Fire on sight — and Occa drags that from "guaranteed KO" to "survives," saving a key priority attacker and flipping the tempo.
Don't give it to anything Fire doesn't hit super-effectively. Occa only halves when Fire is super-effective; neutral or resisted, it does almost nothing.
A 4×-weak mon at half is roughly 2×-weak, so run the number to confirm it actually lives. A strong Fire attacker under sun like Torkoal can overkill so hard that even halved it still KOs — then skip the berry and run an attacking item.