Override field value in view? I have a content type for events. There is an optional field where the user can provide a link that overrides the default link to the event. 

For all of our internal events, the link should take website users to the node of the event. However, for events that are held elsewhere by third parties, the link should take the website user to whatever external link is official for that event. 

I'm having difficulty creating a single link field in a view that is the value of the external link, unless there no value, in which case it falls back to the node link.

I can make a re-written field that is [override_link][path], but that always shows the path, no matter what. I only want the path when there is no override_link value. 

This is in a Drupal 7 site.
The easiest way IMO to do this is to use the views field template. Here is a very detailed and helpful tutorial on DO. You can find the field template's name in the views theming tab under the advanced tab.

In terms of implementation for your use-case, the field's $output in the template would be empty if there is no link, and then you can change $output to be the link to that node (which you should be able to access with $row->nid).