First off, because of the recent introduction of Official Lists/Franchises, OVAs that are related to a series should be added as individual movies, and then be officially connected to the series via a list.

Standalone OVAs should generally be added as individual movies — especially if they are produced that way. If there are a number of movies produced individually that are all connected, they should also be entered individually, and connected with a list. 

The only time an OVA should be added as a series is when the OVA is announced as a series.