An Oster Kitchen Center manual can be ordered on the Oster website under the product support section. The Kitchen Center line has been sold for over 30 years, so it is important to know which model you own before trying to obtain a replacement manual for it.
Alternately, the manual can be searched for on the Manuals Online site and downloaded for free. Another option is to search on the forums of food- and cooking-related sites to see if anyone has the manual for the Oster Kitchen Center model you own. If all of those methods fail, searching on Amazon or eBay may be the best option.