Definition
An outside-in (OI) throw — also called a roll-curve — is shaped so that the disc starts wide of the receiver and curves back toward them as it travels. It is the mirror of the IO: the IO starts inside and fades out; the OI starts outside and fades in. Both are tools for breaking the Mark and for putting a disc on the Break Side.
In Context
The OI is particularly valuable in wind. With a tailwind, a flat throw will speed up and travel further than expected; adding a small OI shape pulls the disc back toward the receiver and counteracts that drift. Into a headwind, an OI on a forehand fades around the wind rather than fighting it head-on, and produces a more controlled arrival.
Like the IO, the OI is built on a Pivot. The release angle and the foot position determine the shape — without the pivot, the throw is a wrist-only effort that does not hold its curve.
See 70s Throwing for the warm-up that keeps OI and IO release shapes dialled in across a season, and Playing in Wind for the specific role both shapes play when the air is moving.