The VirtualBox supports export of its own .vdi files (VirtualBox disk image) to several other formats, including the OVA format (Open Virtual Machine Format), which means vdi to ova conversion is certainly possible. You can achieve this by launching VirtualBox and using the File ► Export Appliance menu option and selecting the virtual machine you want to export.
Open VDI file Open OVA fileFree system virtualization platform for Linux operating system developed by Oracle