OS is operating system. Find out what OS version you are running on the About screen in options. Then find the "latest OS" sticky thread here at blackberryforums.com
for your model BlackBerry. If the latest is newer, consider upgrading. Follow the instructions exactly.
If you are using the latest OS, you have to wipe before you can reinstall. See
How do I wipe the BlackBerry using Jl Cmder? - BlackBerryFAQ for instructions on using JL_CMDER and wiping the OS on the device.
Posted via BlackBerryForums.com Mobile