Short answers, pulled from the story.
OVA stands for original video animation. It refers to Japanese animated films and series episodes made specifically for home video release, without prior broadcast on television or in cinemas. The abbreviation OAV, standing for original animation video, is also sometimes used.
The first OVA to be formally billed as such was Dallos, released by Bandai in 1983. An earlier attempt, The Green Cat from the same year, is the earliest known candidate, but no evidence confirms the VHS tape reached consumers and the series remained incomplete.
The worsening of the Japanese economy in the 1990s reduced spontaneous investment in new OVA titles. The rise of cable and satellite television, which allowed direct broadcast of violent or adult-oriented material, pulled many series that would previously have been OVAs onto regular TV schedules instead.
Bandai Visual reported in a 2004 news release that approximately 50% of its anime DVD customers were men aged 25 to 40. Women made up only 13% of purchasers across all age groups. Nikkei Business Publications separately identified the same 25-to-40 adult demographic as the primary anime DVD buyers.
OAD stands for original animation DVD. Starting in 2008, the term specifically described DVD releases bundled with their source-material manga volumes, distinguishing them from OVAs released as standalone home video products.
OVA episode length varies widely, from a few minutes to two hours or more. A complete OVA series can range from a single episode to dozens of episodes, because the format is not constrained by broadcast scheduling.