The abbreviation "OEI" stands for:

• One-Engine-Inoperative

The abbreviation "OEI" may be met in multi-engine helicopter performance planning and calculations, where different factors that affect performance are examined and the various performance data are extracted for both AEO and OEI cases for the different phases of flight.

OEI (or single engine failure) obviously leads to a limited power situation with potentially half of the previously available power no longer available.

NOTE:
The "AEO" abbreviation stands for "All - Engines - Operative", or "All - Engines - Operating".