Rookie's Blood type.

[EnigmaBiz]

http://http://www.bungie.net/Online/Halo3UserContentDetails.aspx?h3fileid=102523071

Rookie's blood type is 0+...Would this be a requirement of an ODST....(I didn't check everyone)

Your blood type depends on which proteins, called antigens, are on the surface of your red blood cells--A-protein or B-protein--and which antibodies are in your plasma. If your blood cells have A-protein, your blood type is A. If your red blood cells have B-protein, your blood type is B. If your red blood cells have both A-protein and B-protein, your blood type is AB. If your red blood cells have neither A-protein or B-protein, your blood type is O.

Because your blood group is O RhD positive, you are especially important. Around 37% of the population share your group, but what makes you even more important is that, if necessary, your blood can be given to anyone who is RhD positive (regardless of their blood group) - and that's a staggering 83% of people.

[Pulse Cloud]

OMG, that means I could be an ODST! Hooray! (I'm also O+)

[EnigmaBiz]

N/M I guess this is actually common knowledge..

[Imppa]

Negative.
Here's the list of Dare's squad. As you can see, Buck has O-, Dutch B+, Romeo AB- and Mickey A-. You can also see this in-game from the patch in their shoulders, as the pic did.
It's been speculated that Rookie is the medic of the squad (biggest backbag etc.), so maybe O+ would be a requirement to that.
PS: Comforting to see that you still are, Engima.