Hi there,
If I understand correctly omeprazole and esomeprazole (nexium) are chemically similar, though the latter is more used in GERD.
I take omeprazole for years now, and have been using it while on CellCept.
(I stopped taking CellCept after 1,5 year because it didn't work...what if the omeprazole was the cause )

I don't know if you take prednisone or a NSAID (painkiller like ibuprofen or naproxen)? If so, please contact your doc because it's important to protect your stomach.
If not, still contact your doc, talk about other options for your GERD, maybe omeprazole or cimetidine is an option.
(And note, do not use anything you can buy without prescribtion, without consulting your doc first please).