Do oats need a hechsher?

Answer: No, if plain (Star-K, cRc).

Reason: Plain oats have no kashrut concern.

Source:

• Star-K: its list of products that need no hechsher includes pure, raw oats, provided there are no additives and it is not a product of Israel.
• cRc: unflavored oatmeal (including instant) and steel-cut oats are acceptable without a hechsher.

Related questions:

• Does cereal need a hechsher? Yes (Star-K, OU).
• Does flour need a hechsher? No (cRc).

References: Star-K, “Approved Without A Hechsher” list · cRc (askcrc.org), “Oatmeal, unflavored”