I don't know what the Omega back is like -- it might require some
special, hard-to-obtain tool -- so I can't give a definitive answer.
The most-common backs use either a slotted ring -- which can often be
removed with a pair of chain-nose pliars -- or a press-to-fit back,
which is removed with a "case-back knife".
The latter is simply a knife with a very thin blade. You simply press
the knife into the seam, and twist a bit. (Well, that's the theory.
It's sometimes a bit more difficult, and there is the chance of
scracthing the back.)
These are available from Zantech (
http://www.zantechinc.com/store).
They have all kinds of watch-repair equipment and supplies. You'll
have fun browsing the site.
You can get batteries even cheaper than that from Zantech and similar
companies.
Any jeweler who charges more than $5 for a battery replacement --
including the battery -- is gouging. Most batteries cost the dealer
less than 25 cents, and replacing one takes less than five minutes.
That's a gross profit of more than $50 an hour. Not bad.