At the moment a single login account isn't possible in OnDemand.
However, you can log integrate your Google Apps account with your OnDemand instance, so all people within your company can log in with their Google Account, and use this same account for Bitbucket. (Bitbucket also supports non Google Apps accounts).
If JIRA and Confluence are in the same OnDemand instance, then you will only need a single login for both. Fisheye/Crucible will always need their own login now that they've been removed from OnDemand.
There's also a request for that, you can see it here:
Hope this information helps.
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