Hi Diazruanova,
I used to use both together (purely out of not really knowing it was there...) and in my experience the avast! Web shield was FAR superior.
The most I would see from the OA webshield was 'ActiveX content blocked by Online Armor'
Whereas the avast! Webshield well, as DavidR said, the Virus and Worms section of the forum says it all...
-Scott-