OVA means Original Video Animation. Their
continuity depends on the creators. OVA is
just a format. Sometimes a company will
remake an OVA into a TV series (or vice-versa)
if it has been successfull, and changes may 
occur. These can be relatively insignificant, as
in Tenchi, or completely drastic and alienating,
as in Bubblegum Crisis 2040. It's really a 
case by case basis. Hope that's of some help.

-Tom (G.E.)