I'ma long term user of omeprazole & also on domperidone. I get fatigued a lot cud this b vitamin B12 deficiency & if so when would b best to supplement?

Many causes. There's so many causes for fatigue. If this is really troubling you get a check up. B12 deficiency, among other causes are really easy to check. It's unlikely B12 def it's the cause but again anything possible. Stress, other causes of anemia and thyroid are among some things to look into.
See details. Stop the omeprazole. A recent study: proton pump inhibitor and histamine 2 receptor antagonist use and vitamin B12 deficiency jameson r. Lam, mph1; jennifer l. Schneider, mph1; wei zhao, mph1; douglas a. Corley, md, phd1 showed that Omeprazole and other proton pump inhibitors can cause B12 deficiency.
Switch. I would switch to Pepcid (famotidine). The biggest problem with ppi's like Omeprazole is calcium absorption. As far a vit b 12 issues, if it is not absorbing and you are taking large oral doses, you need to get injections. Don't get too concerned with the name brands, most are the same. If you have any concerns go to comsumerlabs.Com, they do quality control as a third party.