O-2 with NASA
 

NASA operated an O-2, N635NA, ex USAF 68-10871, cn 337M0236.
Can anyone shed some light as to when, where and why they used the O-2?

I don't know about that O2, but NASA did use a highly modified 1978 337H for atmospheric research. The aircraft was called the "Cirpas Pelican" and it was used in a project called TARFOX.

http://members.tripod.com/airfields_...elican_OPV.jpg

Now there are two, one X 337H and 1 x O-2.
The O-2 is ex USAF 68-11155, quoting civil reg N11155 but can't confirm this, nor find any record of it.