Ignore all the above as they have it all confused.
With my E-PL5 and with both switched and unswitched OIS I worked out the rules by listening carefully to what the camera and lenses are doing.
The rules are here
http://homepages.ihug.com.au/~parsog/olyepl1/50-epl5-comments.html#stabilisation
But in short.....
If using an unswitched OIS lens then you need Lens IS Priority On in the Custom menus and also IBIS ON, as the IBIS switch is the master switch for stabilisation. Turn off IBIS and you turn off OIS as well. Nice to use at 150mm (with the 45-150mm) as OIS gives a neat stabilised live view without needing to press anything. OIS is preferred to the image blurring E-PL5 IBIS.
If using a switched OIS lens then turn on the switch for OIS and it works and ignores the Lens IS Priority setting and the IBIS setting. Turn it off and the IBIS will take over if it was set On.
The firmware on E-PL5 and later bodies prevents IBIS and OIS from interfering with each other, it's one or the other or none, depending on your settings.
On E-M5 and earlier then the above rules don't apply, they cannot use unswitched OIS at all, plus with switched OIS then you can have both IBIS and OIS working at the same time for a really interesting stabilisation result.
See my timeline to see which bodies handle OIS properly
http://homepages.ihug.com.au/~parsog/olyepl1/80-Oly-timeline.html
Regards..... Guy