// copied from http://edoceo.com/notabene/ova-to-vmdk-to-qcow2
The OVA file is nothing more than a TAR archive, containing the .OVF and .VMDK files. Easy!
Using Evergreen ILS for example:
~ $ file Evergreen_trunk_Squeeze.ova
Evergreen_trunk_Squeeze.ova: POSIX tar archive (GNU)