Hi all, I have just unticked 'Include font style with Copy' under:

>Tools >Preferences >Fonts >Editor

and it has worked for me with the below. I do not have to close Outlook.

PL/SQL Developer
Version 10.0.5.1710
01.92698 - 1 user license
Windows 7 6.1 Build 7601 (Service Pack 1)

Outlook 10 from MS Office Professional Plus 2010
Version: 14.0.7116.5000 (32-bit)