It is not, and I'm not aware of any plan to make it available.
You can integrate OnDemand with a remote Zephyr though - see https://confluence.atlassian.com/display/AOD/Connecting+Zephyr+with+JIRA+OnDemand
Currently Zephyr for JIRA is not available for JIRA OnDemand. However, it is in our road map to make it available for JIRA OnDemand. We do not have any time frames yet to release as to when it will be available. However, we post all of our upcomming announcements to our blog:
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