Let us suppose that I have created a command queue with flag “CL_QUEUE_OUT_OF_ORDER_EXEC_MODE_ENABLE”, some buffer and two kernels. If I pass same buffer to both kernels and enqueue them, will they execute simultaneously? (we will mean, that both kernels have empty wait lists of events)

They are allowed to execute in parallel in that case since there are no event dependencies, thus might or might not actually run simultaneously.