Several types of feeds dont work in OW, including the O'Reilly feeds:
http://www.oreillynet.com/pub/feed/13
and others, for example:
http://pyinsci.blogspot.com/feeds/posts/default
just results in a page of text.
There are lots of RSS-related bugs -- I have reported 3 in the past week. Hopefully they are being addressed in 5.6.
Thanks.