Any new OU pathways you add will take affect the next time a full sync is run. Within the site settings for Active Directory you can set a full sync to run either once, twice or three times a day.
Alternatively, you can click OK at the bottom of the Active Directory page and that will trigger a manual sync to start.
To set how many times a day Active Directory Syncs, go to the Oak Menu > Configure > Site Settings and then click onto the OIS menu on the left hand side. Here you can set the synchronisation intervals.