Just a little warning to anyone planning to upgrade their OpenX to version 2.6 (2.6.1).
IF
You have a large amount of data stored in the mySQL database the upgrade might not work. I tried several times and had to restore the database each time with a backup.
In the end I had to truncate all larger mySQL tables which meant I lost a lot of data. However once they where smaller than 100,000 entries the upgrade went fine. (I upgraded from 2.4.7).
Whatever you do, do a database backup!!