LogMeIn Hamachi offers a free VPN for 5 computers. This means that these 5 computers are always directly connected when you 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, you need a fee-based tariff. More on that http://vpn.net
If you have now created a VPN, you can add the individual PCs to your VPN. To the Raspberry Pi and thus also the OSPi to add, I found the following instructions:
Raspberry Pi: Install Hamachi
The ARMHF version is the Raspberry Pi compatible version.
.
.