I agree with sded's post as I have a very similar security setup on 3 machines except sded is using OA ++ (without the AV) and I am using OA Premium [FW only] (along with the other security software mentioned in his post).
It is important to make exclusions and there are no problems. I have used and tested these software for several years now with no conflicts with multiple version changes and uninstalls/installs and upgrades.
Also as mentioned, OA webshield works entirely different from Avast.
You can if you want have OA and manually disable the firewall (FW), just like you can in AIS.
Both OA and AIS offer different products that you can customize, so I do not see this as overkill. With proper exclusions, this is layering of security. Should a conflict develop, perhaps due to a definition or version upgrade, then troubleshooting will need to be done.