Please download the OSM documentation from edelivery site, in which you can refer to Concepts Guide. Moreover, there is additional document inorder to understand the cartridges developed for O2A. Please refer to these two docs to get all concepts.
I would also refer you to the OSM Foundations I, II and III training courses available through Oracle University. These courses should be considered mandatory for anyone working to develop solutions based on the OSM applications.
Here is the link for edelivery I mentioned https://edelivery.oracle.com/. Login & select "Oracle Communication Applications" & Platform as "Microsoft Windows 32".
From the results displayed, download docs from these two location
1)Documentation for Oracle Communications Order and Service Management Media Pack v2
2)Documentation for Oracle Communications Order and Service Management 7.0.3 Cartridges for Oracle Application Integration Architecture Media Pack v1