Oracle Virtualbox by default has Right CTRL key assigned as the HOST key. HOST key is a special key on your keyboard which Virtualbox reservers to return ownership of keyboard and mouse to your host operating system. This however can be changed to the one you prefer. Say for instance, if you want to change it to Left ALT (same as VMWare).

To change the HOST Key,

1. Launch Oracle Virtual\box Manager.
2. Click File – Preferences.
3. Click Input in the left pane and click “Virtual Machine” tab on the right.
4. Click on the Shortcut field for “Host Key Combination” and click the erase icon that gets highlighted.
5. Now click the Shortcut field again click the key you want to assign (say “ALT”) and click OK.