OAD | Japanese with Anime

An OAD, or ODA, is an anime bundled as a bonus disc into a limited edition manga volume that never aired on TV or anywhere else.

If an anime is first sold as DVD/Blu-ray, then airs on TV, it's an OVA. If it airs on TV first, then is sold as discs, it's not OVA. If it's sold as disc, and never airs on TV, it's OVA. If the anime airs on TV, and then it's sold on discs, but the discs also include bonus content such as an extra episode, the anime is not OVA, but the extra content is OVA. If an anime is first available on the internet, it's ONA. If it's first available as a bonus in a limited edition manga volume, it's OAD.

Source: japanesewithanime.com (CC BY-SA 4.0)

Definition

An OAD is an anime bundled into a manga volume, also called a tankoubon 単行本, as a physical disc, as a DVD or Blu-ray.^{(dic.nicovideo.jp)}

Typically, a manga or a light novel that has a manga adaptation gets an anime adaptation that is broadcast on TV, and the studio adapting the series also animates an OAD that doesn't air on TV, but is instead included as a bonus in a manga volume released after the TV anime ends.

An OAD is often just a single episode of extra content, and, in some cases, it's not even the same length as the 20 minute TV episodes. It can be a short of 10 minutes, for example.

The manga volume that includes an OAD is a "limited edition," gentei-ban 限定版, and is way more expensive than the normal edition.

The seventh volume of Asobi Asobase あそびあそばせ has a limited edition with a 13 minute OAD. The normal edition is sold for 660 yen ($6.19), while the limited edition is sold for 4900 yen ($45.93), over seven times more.
For comparison, the second DVD of the anime adaptation, containing episodes 4, 5 and 6, a total of 60 minutes of anime, is sold for 4424 yen ($41.47).

Acronym

The acronym OAD stands for Original Animation Disc (or DVD). It's also spelled ODA, Original Disc Animation.

vs. OVA

The difference between OAD and OVA is that OAD is a type of OVA. OAD specifically refers to OVA bundled in limited edition manga volumes.

An OVA is an anime that was originally released as a disc, rather than airing on the TV first. The term often, but not always, refers to bonus episodes added on the DVD releases of TV anime. OAD is the same idea, but for manga volume releases instead.

Some anime database websites, like MyAnimeList, do not have a specific category for OAD, and just categorize them as OVA instead.