Rilezu cels are post-production cels.  They're
handpainted (dunno about hand-inked though,
probably not) from the douga (sketches) of
digitally painted shows.  OVA stands for Original
Video Animation (sometimes OAV, Original Animation
Video), and is simply a direct to home video
release.  The term, OVA, in relation to cels in
general means nothing.  When narrowed down to
specific series, such as Fushigi Yuugi, or
Slayers, it can mean a vastly different art style,
and perhaps even different production teams!