OFDM divides the signal in frequency domain into orthogonal and relatively narrow subcarriers where fading can be considered to be flat Flat fading is not associated with delay spread and is corrected in frequency domain

Delay spread caused by multipath is associated with frequency selective fading and in OFDM it is absorbed by the cyclic prefix which isolates consecutive OFDM symbols

How would delay spread show up in the frequency response of the channel? would it show up like "nonflat" fading or a notch within the relatively small bandwidth of some subcarriers?

i.e. it would be wrong in this case to consider the affected subcarriers still experiencing flat fading?