LogMeIn Hamachi offers a free VPN for 5 computers. This means that these 5 computers are always directly connected when they are online, no matter where they are. In addition to PCs and smartphones, this also includes the Raspberry Pi. If you want to network more than 5 computers, then you need a paid tariff. More on this http://vpn.net
Once you have created a VPN, you can add the individual PCs to your VPN. To add the Raspberry Pi and thus also the OSPi, I found the following instructions:
Raspberry Pi: Install Hamachi
The ARMHF version is the Raspberry Pi compatible version.