It WILL faint in this case. If Rampardos OHKOs Blissey:
0 Atk Rampardos Head Smash vs. 4 HP / 0 Def Blissey: 1155-1359 (192.5 - 226.5%) -- guaranteed OHKO
0 Atk Rampardos Head Smash vs. 4 HP / 252 Def Blissey: 541-637 (90.16 - 106.16%) -- 37.5% chance to OHKO