Apparently what OE puts on the clipboard for RTF isn't identical to what it's displaying in its browser. A couple possibilities:
1) Configure OE to only show messages as text (Tools | Options | Read | Read all messages in plain text). Then, it will only put text format on the clipboard when copying. Plus, it's a good way to avoid potential security holes [1].
2) Install PureText [2] to enable a system-wide shortcut (i.e., Win+P) for pasting as text no matter what format(s) are on the clipboard.
[1]
http://www.google.com/search?q=outlo...+text+security
[2]
http://stevemiller.net/puretext/