OEM is just original equipment manufacturer, so the stock ecu.
There is a point where the stock ECU can no longer run the engine over its entire operating range, and this is typically once you exceed a valve overlap value or change the induction method. A standalone then is require, not because it offers dramatic performance increases, just the ability to control the engine in a manner which you can dictate as OEM ecu's are setup to run speed density and have their own parameters which the engine is 'supposed' to run in between.
However, you wont notice any performance increases between an OEM and aftermarket ECU on mild level modifications (which is what 90% of people run). The added benefit is complete stock running of supported systems such as aircon, dash displays, warning lights, fault logging and diagnostics, cruise control etc etc.