It's an issue with the bridged network driver, the driver should be fixed by VirtualBox. Install VirtualBox without the bridged network driver, use NAT instead and there's no conflict with OA.
Virtualbox forum seems down at this moment. If it's up again, read this topic: https://forums.virtualbox.org/viewtopic.php?f=6&t=48044
Disabled aero is caused by the colored border OA shows on programs set to run safer. If sandboxie's restricted rights are used, it's detected by OA as 'runsafer' and the colored border is shown.
Disable the option to show a colored border in OA (Programs > Options) and there's no issue with aero.