where can we find and watch ONAs?
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.