First you need to consider that changing your scanning methods won't change already existing data in your database. If you would like to restrict the number of assets in your database, you first need to change your scanning methods and afterwards delete the assets you don't require. Instructions on how to delete assets can be found
here.
Secondly, all scanning methods which you can set up under
Configuration\Scanning methods work independently from each other. If you set up OU filters for
Active scanning, they will only apply for Active scanning.
IP range scanning for example still continues scanning the IP ranges you submitted under section IP Address Range scanning.
Third, Active scanning only picks up computers which recently have been logging on to your domain controllers. Computers which didn't log on during the last 7 days or which logged on to domain controllers that can't be reached from your Lansweeper server won't get scanned. Lansweeper needs to have a connection on port 389/TCP to all relevant domain controllers.
If you would like to scan all computers having an account under a specific OU, you might rather submit the ADSI path of that OU for
Scheduled computer scanning. Scheduled computer scanning only needs to contact one domain controller, not all.