Well this is a bit of a stretch on part bridging two separate eras and game systems, but he's my take. In the old FASA Starship Combat Simulator, each the PC crew had a function in combat. The Security Office in my house rule rolled his Security Prodecures skill reduce incoming casualties when the ship took damage that caused casualties (The doctor reduced casualties that got past the Security Officer). This is of course based on the TOS/TMP era duties of the Security Officer. In the TNG the Security Officer is rather busy with other functions, so as black campbellq suggests, maybe you can have the XO roll ro reduce incoming casualties?
Another option would be if the XO isn't doing anything else in the turn, he can make his Starship Tactics, or Security Procedures, or Admin, or whatever roll to reduce the multi-action penalty on the Captain for that turn. IE if the XO makes his skill test, the captain gets one extra action without a penalty. This is a very accurate portrayal of what XOs do during combat. They assist the Captain in commanding the ship.
"For to win 100 victories in 100 battles is not the acme of skill. To subdue the enemy without fighting is the acme of skill." Sun Tzu - The Art of War