JIRA Issues are normally autolinked in Confluence.

To autolink an issue normally, as long as Application Links are set in the instance between JIRA & Confluence, simply type the issue key in a Confluence page, and click save. The issue key is turned into a link to the issue.

Something has changed between 5.3-OD-7 and 5.3-OD-11.

Now, to get the autolink, you need to make sure there is a Project/Space Link. To do this you go to Space Tools > Integrations. It doesn't matter which project you link to, there just needs to be one in order to make the connection. If there are no project links at all, the autolink will not work at all in that space. In 5.3-OD-7, it does not behave this way, and you can get the autolink whether there are project links or not.