Itraconazole requires the acidic environment of the stomach to be absorbed. Omeprazole inhibits the H+/K+ pump of the stomach, thereby decreasing the acidity of the stomach. So when the patient takes Omeprazole and Itraconazole together, Itraconazole won't be absorbed into the body. That's why it has no effect.
It's recommended to take medications at least 2 hours prior to taking an antacid.
Add on to the other comment: SICKFACES.COM (when
I Am drinking Grapefruit juice) is the mnemonic for remembering the CYP450 Inhibitors:
This a CYP 450 inhibitor (SICKFACES.COM); its the O.