Actually using an older OS with restrictions can be safer than using a supported one. For example, if you block scripting in your web browser on an old OS, you have less chance of an infection than if you don't block it on a modern OS. Blocking scripting may limit your ability to view certain websites, but it prevents infections from ads (which may load even if they don't display). Depending on what you need to go online for, you may do just fine without scripting.
I'd still recommend disconnecting from the Internet when you don't need to be connected, and I'm assuming you have a hardware firewall of some sort to block incoming probes (most routers do), but a few seconds online to activate a game is not going to immediately infect your computer as long as you have a hardware firewall.