A trial, or beta, version, of Leopard will be available in June at the Apple developer conference.
The delay has been caused by the demands of the Apple iPhone which is intended to go on sale in June.
To ensure the iPhone hits its target, Apple has assigned some OS X engineers and quality testers to that project.
In a statement, Apple said the sophisticated software on the iPhone had forced it to make some difficult choices about where to concentrate its development and engineering efforts.
Apple said that the features in Leopard will be complete by the time of its Worldwide Developers Conference in June but the final polish on the product would be missing.