I'm using omeprazole mostly because I can't get antiemetics. Just to be clear though, it's not a replacement for an antiemetic. It's a PPI, meaning it reduces stomach acid over time. So, yeah. It's not going to stop my nausea or prevent vomiting, but it can help keep the stomach environment less hostile, especially because I'll have fasted.
You can take it the day of without a loading period because it still reduces acid secretion after one dose, but it's more effective if you take it consistently for a few days. That said, in a pinch, one dose is better than none.
Because I have nothing else, omeprazole + fasting + cold water + no gag reflex is about as stable a setup as I'm going to get regarding vomit prevention haha.