OVA’s (Original Video Animations), ONA’s (Original Net Animations), and Specials (episodes that are aired similarly as the original broadcast but are aired separately) are all permitted on AniList.
An OVA is a title that released directly to home video via something like VHS, LD, DVD, BD, etc.
For an OVA series to be combined into a single entry, it should be released on a semi-regular schedule and/or should have a shared continuity (see Example 3).
For OVA’s that are attached to a parent series, they should be combined only if they apply to the same parent series (see Example 1).
Example 1:
Yahari Ore no Seishun Love Comedy wa Machigatteiru. OVA, an OVA alongside the first season of the show, was bundled with a limited edition console game.
However, another entry would be made if the OVA in question has a tie-in to a specific season, such as Yahari Ore no Seishun Love Comedy wa Machigatteiru. Zoku: Kitto, Onnanoko wa Osatou to Spice to Suteki na Nanika de Dekiteiru.
Example 2:
Kono Subarashii Sekai ni Shukufuku wo! OVA, an OVA alongside the first season of the show, was bundled with a volume of the light novel.
However, Kono Subarashii Sekai ni Shukufuku wo! 2 OVA, is its own separate entry as it was tied into the second season of the airing TV show.
Example 3:
The show Kidou Senshi Gundam UC was aired in a set schedule, with each episode following a singular continuity, so it is combined.
An ONA is a type of series that airs online for its original broadcast release.
A work will be labeled special if the production company labels it as such. A special is also a work that is uniquely different than it’s parent series, whether that be a picture drama, a short, a pilot, etc.