Reference: Regulation (EU) No 965/2012 on Air Operations, Annex IV (Part CAT)
In the AMCs to the Regulation (EU) No 965/2012 on Air Operations (available here), it is recommended, for commercial air transport operations, to carry an automatic external defibrillator on aeroplanes required to carry an emergency medical kit (those having a passenger seating configuration of more than 30 seats when any point on the planned route is more than 60 minutes flying time at normal cruising speed from an aerodrome at which qualified medical assistance could be expected to be available) under certain conditions.
Namely the acceptable means of compliance to the rule concerned (CAT.IDE.A.225), listing the content of the Emergency Medical Kit, recommend operators to determine through risk assessment the need to carry the defibrillator. So there is no strict requirement for operators, but only a recommendation based on the result of a risk assessment.
The full text of the AMC is available here.
The above is in line with the current ICAO Annex 6 recommendation, which reads as follows:
ATTACHMENT B. MEDICAL SUPPLIES
Supplementary to Chapter 6, 6.2.2 a)
1.2 Based on the limited available evidence, only a very small number of passengers are likely to benefit from the carriage of automated external defibrillators (AED) on aeroplanes. However, many operators carry them because they offer the only effective treatment for cardiac fibrillation. The likelihood of use, and therefore of potential benefit to a passenger, is greatest in aircraft carrying a large number of passengers, over long duration sector lengths. The carriage of AEDs should be determined by operators on the basis of a risk assessment taking into account the particular needs of the operation.