Unlike Nikon for example who have kept the same t-rings for many of their current DSLR cameras, Olympus have a range, that have changed over the years. So you need to be sure you are buying the right one. Hopefully this guide will help
For older film cameras such as the OM-1 , OM-2 etc you will need the standard Olympus mount that is still readily available
As Olympus turned digital, the 4/3 mount was introduced to work with modern crop sensor cameras. Though the older OM lenses can still be used. The 4/3 t-rings are designed for cameras such as the Olympus E-500 , E-510, E-520 etc
Lastly as the current range of Olympus compact DSLR cameras, you need the micro 4/3. This compact design means it will not work with older lenses. As well as the current range of Olympus cameras, Micro 4/3 also work with many Panasonic cameras. If in doubt, ask your local dealer for advice
T-rings, T2 mount can be purchased from Northern Optics HERE