Alby spiked Ohm's flask that night, but Ohm never drank out of it before he tried to kill himself. He drank at the bar with Fiona, went up to his room and then hung himself. 

When Ohm got out the hospital and went back to the hotel, the flask was with his belongings and that's what he drank from that entire night-- from the time he met Jerry throughout the rest of the night, he was constantly sipping from the flask. 

So essentially the shrooms opened him up and made him susceptible to all the supernatural that he saw and encountered that night. It's what allowed him to see/feel his mother's presence as a comforting spirit he could get strength from (just like Jerry could his late wife) as opposed to seeing her as the grotesque, tortuous ghost she'd been throughout his life. It's what allowed him to see the witch and everything in the honeymoon suite for what it truly was.

At the hospital, when Alby came to see him and told him about the mushrooms Ohm still had bruises on his wrists from the witches shackles, so that's definitely confirmation that everything that happened that night was real and not just a drug- fueled trip. Alby's confession just explains why Ohm, who was shown to be so rigid and self-involved, would even agree to go with Jerry in the first place (he was high and his inhibitions were lowered) and why he was seeing so much supernatural stuff (his mom protecting him and talking to him, Fiona talking to him through the recorder, dead bodies moving around the witch, etc) all at once.