Hi jmyva,
I don't have a Veo 250, but have an Oceanic Versapro, which has many similarities to the 250. So, I'm not sure that my answer will apply, but as nobody else has jumped in, I'll give it a go.
The Fo2 default (on the VP) functions as follows: (if the setting is "on") after a nitrox dive, the computer will automatically default to a 50% oxygen setting for the next dive, unless the diver resets the F02.
IIRC, this has the computer working, in theory, with an impossible gas situation:where the computer is simultaneously working NDLs and N2 loading on the basis of air,(21% O2) while running the O2 clock, and setting the MOD as if the breathing gas were 50% O2. The rationale is that, in the interest of safety, the computer will act in the "safest" manner possible-giving the shortest NDLs,( to hopefully reduce the risk of DCS) along with the shallowest MOD and the quickest O2 loading-to help lessen the risk of Oxtox.
If you set-up the computer to have the F02 default "off", the O2 setting will remain as you left it on the previous dive.
That is how I have my VP set.You just have to remember to reste the Fo2 for each dive.
Hope this helps, but as I said , this is for the VP, not the Veo 250.
goord luck,
Mike