When I Googled for "OPAC Data Model" I didn't get any useful up-to-date Results.
The only Data Model-type item was from the link I have described as the Data Domain below.
However, this was written in 1996 and I have updated it appropriately.
This is definitely an interesting area, where a Survey of the current State-of-the-Art would be very worthwhile.
A good deal of attention is being given to Web Services and to ways of linking up different Lending Institutions.
If you are a student given this as an assignment then you are very fortunate and if I can help you, just let me know.