OEI-(cruise) speed is a fixed value and subject to approval by the authority and can be used for pre-flight planning. This speed is based on the following;
• international standard atmosphere,
• level flight, and
• FL170 or maximum level with OEI according to the AOM/AFM or OM.
Note: As the critical fuel scenario according to international regulations must cover one engine out and decompression (i.e. uncontained engine failure causing structural damage), FL100 must be considered. Therefore the authority may approve an OEI which is higher than optimum at this level to guarantee maximum diversion distance.
This speed shall be used to determine the area of operation, any dispatch limitation, calculation of single engine fuel and oil supply and to establish the level off altitude. This level-off altitude (net performance) must clear any obstacle en-route by margins.
In case of an actual OEI situation in-flight the manufacturer recommends to fly at optimum, weight dependant speed using AOM/OM tables or according to FMS calculation.
Source: ICAO