Flaked Oats are unmalted, but they aren't raw the way a bag of grocery-store oats is raw — the flaking process itself, steam and pressure followed by rolling between heated rollers, gelatinises the oat's starch before it ever reaches a brewer. That matters more than it sounds: raw, unflaked cereal grain needs a hot cereal mash to gelatinise its starch before a normal mash temperature can access it, while pre-flaked oats mash in directly alongside everything else with no extra step required.
What they actually do to a beer
Oats contribute a distinctive silky, almost creamy mouthfeel that no other common adjunct quite replicates, along with a soft, mildly sweet grainy character. That silkiness comes from a combination of beta-glucans and lipids in the oat itself, and it's the specific texture reason hazy/New England IPA recipes lean on flaked oats so heavily — the style's soft, rounded mouthfeel is at least as much about the oats as it is about the yeast strain or late-hop technique.
No diastatic power of their own
Like every unmalted adjunct, Flaked Oats contribute nothing toward a mash's own conversion capacity — they need a base malt with enough enzyme margin to convert their starch alongside its own, which at up to 20% of the grist is well within reach of any standard high-diastatic base malt like Pale 2-Row or 6-Row.
Where the 20% ceiling comes from
Push oats too far past their conventional ceiling and the beta-glucan load can thicken the mash enough to slow or stick a sparge, similar in kind to the lautering risk high-wheat or high-rye mashes run, though oats are generally more forgiving than rye specifically. A thorough mash-out and a reasonably coarse crush help keep a high-oat mash moving.
Haze, on purpose or by accident
Flaked Oats contribute meaningfully to a beer's haze, both directly through their own protein and indirectly by interacting with hop-derived compounds in a hazy IPA's characteristic cloudy appearance. In a style built around that haze, this is exactly the point; in a style meant to finish bright, an oat addition that's larger than intended is a common, under-recognised cause of a beer that won't clear the way a brewer expected.
Gum content and clarification time
Oats carry more of the same gum-forming compounds responsible for a thick mash than most other adjuncts short of rye, and that gum content doesn't entirely disappear once fermentation is done — a heavily-oated beer can take noticeably longer to drop bright in the fermenter or during cold conditioning than an equivalent all-malt batch, independent of the haze a hazy IPA is deliberately built around. Brewers making a clear-finishing beer with a smaller oat addition sometimes find it needs a few extra days of cold crash to fully settle.
Typical use
Hazy IPA and New England IPA are the styles most associated with flaked oats today, though the ingredient predates that trend by a long way — oatmeal stout has used flaked or malted oats for body and mouthfeel for well over a century, and the two styles put the same silky-body property to very different flavour purposes.
What else delivers a similar texture
Malted oats (oat malt) can stand in where a brewer wants a similar texture contribution with some of its own diastatic power, though it's less widely stocked; flaked barley is a reasonable body-and-head substitute where oats specifically aren't available, though it won't replicate oats' particular silky character.