OCS has a very robust plugin mechanism which is the recommended way of extending functionality to communicate with another application. Depending on what your requirements and goals are, your best bet with OCS would be to familiarize yourself with the technical documentation, and implement the integration as a "generic"-class OCS plugin, acting as a bridge to your other accounting application. How realistic and cost-effective this is depends heavily on your development resources, the complexity of your required functionality, and what the API to your other application is like.
We are still working dilligently on improving the quality of and access to documentation, but to my knowledge we don't have a fully customized OCS technical reference yet. However, I'd refer you to the OJS technical reference (http://pkp.sfu.ca/ojs_documentation
), which, although not identical, is similar enough to show you what's involved.
Hope this helps,