Setting the takeoff altitude correctly could be the difference between a successful and a frustrating end of the flight.
An Oudie 2 without being connected to an external source of pressure data will only use GPS altitude. There is little to nothing you can do with that and this is the reason why such flights are ok to record the joy and excitement about the flight but are insufficient to provide proof or staying outside official airspace regardless if this is in a competition or on a record flight.
Oudie IGC and Oudie 2 connected to an external source of pressure data will record extremely accurate altitudes. The altitude in the Oudie IGC and all other IGC approved flight recorders is recorded at the standard pressure of 1013.25 hPa. In order to know you current altitude above mean sea level and your current flight level altitude the altitudes need to be corrected to correspond with the real world.
There are several ways how Oudie / SeeYou Mobile can correct the takeoff altitude. It will typically assume that the pressure recorded by the pressure sensor is relative to the standard atmosphere pressure of 1013.25 hPa (but it is even possible to correct for that, see below). It is quite possible to correct the altitudes to QNH automatically with the data which is available to Oudie. If you get the from an external source (such as the ATC tower or the competition task sheet) then it is possible to enter the values manually as well. In order to correct the takeoff altitude you need to go to Menu > MC & Alt. dialog.
By default the Auto QNH checkbox is checked. Auto QNH will try to correct the takeoff altitude to your current altitude by searching for a nearby airport or by using the Ground elevation at your location or (if all else fails) using a very much averaged GPS altitude (but the latter is only a fallback and should not normally happen since there is a worldwide terrain database already installed on each Oudie). Auto QNH setting will give you a very good idea about the current pressure level and it works automatically without your input.
If you want to change the altitude manually you should disable the Auto QNH navbox and enter the takeoff altitude manually. If the Linked checkbox is checked (which it is by default) then the pressure level will increase accordingly.
Typically in a competition you could get a complete set of data from the competition organization. In this case remove the Auto QNH checkbox AND the Linked checkbox. Enter the takeoff altitude AND the QNH pressure given from the competition then press ok. This way you will see exactly the same altitudes during your flight as the ones that you will be scored by later in the evening. DO NOT set the "linked" checkbox back in this case because it will change the pressure level back to the "linked" value.