Change the line
;(gtk_accel_path "/Buddies/Show/Offline Buddies" "") to
(gtk_accel_path "/Buddies/Show/Offline Buddies" " o")(It is important to remove the semicolon at the beginning of the line which disables the whole line.)
I chose Ctrl-O (Conversation/Get Info gets still activated with the same shortcut in a conversation window.)