If you are referring to anonymously accesible Confluence (wiki) space, you can achieve this.
Grant anonymous user the 'can use' permission in the global permission setting at https://domain.atlassian.net/wiki/admin/permissions/globalpermissions.action. Then, in your space, grant 'view' permission for anonymous user, for example at https://domain.atlassian.net/wiki/spaces/spacepermissions.action?key=SPACEKEYHERE
It is similar to http://confluence.atlassian.com/display/DOC/Setting+up+Anonymous+Access. Is this what you are looking for?
In the URL https://domain.atlassian.net/wiki/admin/permissions/globalpermissions.action, change the domain.atlassian.net to your own OnDemand domain.
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