The only Virtualbox operation for an OVA file is to import it. An OVA file is actually just a compressed and zipped folder. I believe it used the gzip compression algorithm. What this means is you can actually unzip an OVA file and access the vmdk file directly. The "Import" command in Virtualbox simply unzips the OVA, adds the VMDK to the Virtual Media Manager, and imports the configurations.
To answer your question directly, you want the "Import" function, then once the VM has been imported, you can "Power On" the VM and log in.