OPEN SYSTEMS Accounting Software (OSAS) is a business accounting software package for small- to medium-sized businesses using the Microsoft Windows, Linux, and Mac OS X operating systems. First sold in 1976, it has been actively developed and sold since then by Open Systems, Inc. of Shakopee, MN.OSAS is currently programmed in the Java-based BBj language developed by BASIS International. The Java implementation allows installation of OSAS on a variety of platforms.