Flash with custom recovery. This is a beta run also. Speeds above 800mhz may be unstable for many people so find the speed that works best for you as all processors are unique due to construction imperfections.
Not all phones can OC and can handle the same speeds. Your phone is unique. Some can go up to 864mhz stable some can only do 729mhz and some can't at all.
Overclocking must be done with setcpu. After each boot/reboot the max is set to 600mhz to keep you from getting in a boot loop with an unstable speed for your device. To change it after reboot you must move the slider in setcpu to get the overclock freq to set above 600mhz. Do not use the set on boot feature of setcpu.
Download Optimus S GNM OC Beta
Please make a nandroid backup prior to flashing so you have a restore.