John
I think that's a typo. It has a CMOS sensor, not a MOS. See here
http://www.dpreview.com/products/olympus/slrs/oly_epl1s
The camera is a four-thirds system, just like my Panasonic G1. The format is 4:3 (4000x3000 pixels) which gives you the 12MP. The sensors are 17x13mm - slightly smaller than a standard DSLR, such as the Rebel, or the D90. I find the image quality of mine excellent for a small camera. Why have one? Well, it's great for me as I don't always want to lug the 5D around, and it's such fun to handle and shoot with.
Don't be put off by people who say the electronic view-finders (EVF) are no good. Bridge cameras used to have EVF and they were bad, but these 4:3 are much better. There are some advantages to losing the mirror of a DSLR - smaller camera, less weight, smaller lenses (the crop factor is 2x). But the biggest gain is that you can see through the viefinder the changes made by altering camera settings such as exposure, which you can't do on a DSLR.