Technically, yes. Realistically? It probably depends on the horse. Absorption will be less if there’s food in the stomach, the question is, is it enough less to make a difference for that horse.
I would ALWAYS choose diet over drugs. Outlast is proven to raise pH above 4.0 (the magic threshold) for up to 6 hours, and for a lot of horses, using that for their work (ie feed it right before you get him out to groom/tack up/warm up) is all they need. For others, an additional serving is often all they need.
Adding omeprazole to Outlast is doing 2 different things. Personally, I would not use the drug if you find the Outlast is going all that you need it to.