The resonant behavior of delayed oscillators is studied using two simple prototype equations similar to that used by Suarez and Schopf. One prototype equation has a periodic modulation of simultaneous feedback, and the other prototype equation has a periodic external forcing term. The periodic modulation yields even-multiple resonance to the modulation periods, while the periodic forcing results in odd-multiple resonance to the forcing periods. The reason why these two-types of resonance occur in each system is explained. The key mechanism for the resonance is that a positive simultaneous feedback for small amplitudes sets a threshold. Only when the sum of non-simultaneous terms is larger than the threshold, a phase reversal can take place. The implications of El Niño elimination due to annual cycle and potential importance for the decadal variability are discussed.