According to the Olympus' User Manual for the OM-2s:
http://www.olympusamerica.com/files/OM_2S.pdf
This camera uses 2 SR44 or LR44 1.5 Volt Silver Oxyde Batteries. The LR's are Alkaline.
For example the Duracell D303/357B 1.55V Silver Oxide replaces the SR44, Other brand equivalents:
Mitutoyo Part# 541980
Duracell: D357/303
Rayovac: 357
Panasonic: SP357
Renata: 357
Sony: SR44W
Toshiba: SR44W
Varta: V357
Maxell: SR44W
GoldPeak: 357
Timex: J
AWI: S05
Bulova: 228
Seiko: SR44W
Energizer 357 or 2032
Energizer EPX76BP Photo Battery
Other: EPX76/S76, GS13/GP57
These batteries are widely used in many devices including hearing aids and should be available at CVS, Walgreens or Walmart if not in the Camera department look in the watch department.
You might also be interested in this article:
http://www.mir.com.my/rb/photography/hardwares/classics/olympusom1n2/shared/rick_oleson/index.htm
Where to buy online:
http://www.energizerbatteryfinder.com/results_new_2.asp?form=oem&N=0&Ntk=All&Nty=1&Ntx=mode%2Bmatchallpartial&Nu=p_prodcode&Ntt=357&brand=Duracell&x=14&y=4
http://www.zbattery.com/d303-357.html
http://www.amazon.com/Duracell-LR44-1-5v-Alkaline-Battery/dp/B00006JPHE