Basic Resources
Official OCL language specification (Version 2.2)
OCL Book
The book 'The Object Constraint Language: Getting Your Models Ready for MDA, Second Edition' provides a thorough and up to date introduction to the topic
Definition of OCL in Wikipedia
Software
There is a comprehensive List of OCL Tools at the Dresden OCL Portal.
Miscellaneous
Blog postings on OCL on modeling-languages.com on Jordi Carbot's site modeling-languages.com.
Precise UML (puml) mailing list
The puml mailing list is a great place to discuss OCL issues online.
OCL tutorial on parlezuml.com
This is a very good introduction to OCL (pdf) written by Jason Gorman.
Article by Gary Pollice on OCL
This article demonstrates the added value of using OCL to enrich UML models and also makes a suggestion how to use OCL to specify testcases.
