There's no OC'ing the i3-550 except for a slight bump in the bclock. Won't make any noticeable difference. Best you can do for fps increases is upgrade the gfx card to something a bit faster that won't be too badly bottlenecked by the slow-ish dual core. Maybe a RX-460 or possibly a GTX 960.
Then turn off/disable everything unnecessary running in the background with Win. Run CCleaner to clean up resource-wasting crap and registry errors. Maybe that will free up some CPU clock cycles. If your biggest performance drop is in multiplayer gaming on a busy server, the CPU is the culprit. You may be able to upgrade to a slightly faster quad core if your motherboard supports one.