I've had no problems using Avast 4 and OA (
trial or Premium -- no antivirus). There have been no problems with the Web Shields as they act slightly differently. OA has an automatic whitelist, so no need to update. The blacklist can be downloaded but updated periodically; OA's software is simple to use and has a newbie or advanced mode as well as a banking mode. Their forum is also as friendly as this one
, and there are many Avast users on the OA site as well. If you know your machine is completely malware-free, when you install OA, you tell it to "trust everything" on your machine so there will be no software conflicts...but you still need to add Avast as a trusted exclusion and vice versa. If you're not sure you are malware-free, OA can check for you by not checking off "trust everything." I also run MBAM and a few other malware (on demand tools), so make sure your machine is clean prior to "trusting everything" as well as keeping Avast up to date and scanning and you should be fine.