If a drive manufacturer markets a drive as 120GB, then there are 120GB's worth of writable sectors on it. SSDs often include space above and beyond this for wear leveling, but the extra space isn't available for use as storage. If OCZ isn't doing this then they're being awfully shady, IMHO.
When you partition and format a drive, some of the space is used for file system overhead.
And while drive manufacturers think 120GB = 120,000,000,000 bytes, Windows thinks 120GB = about 128,849,018,880 bytes (120 * 1024^3). So when you look at the drive in Windows it will appear to be only around 111GB.