Great question — answering as the UphillTech team:
A lot of ONAs get released either through the Bandai Channel or on Nico Nico Douga . Releases on the Bandai Channel are usually region locked and sometimes, they're re-release them on their Youtube channel (which, sometimes, is also region locked). Nico has an English version of the site which makes it a lot more accessible. Both sites are free, Nico requires registration to view most videos and there is also a "Premium" member access.
Full answer by Jon Lin: https://anime.stackexchange.com/a/8673 (CC BY-SA 3.0).
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