So far as I know, all the early Omega and Pulsar watches used 355 size batteries, that are no longer manufactured. However, it is common practice today for owners to use the smaller, easily available, 357 size (made by Renata, Energizer, Maxell etc..) along with a plastic 'spacer' so the batery fits snugly in place .
Spacer kits can be bought for under $10, but as the spacers in question are no more than plastic sleeves made from cuting a thin slice off of a piece of plastic tubeing, you can make your own. For more info, click the search button at the top of the page, i'm sure this subject has also been mentioned previously :D