[minimal]

O₂NONO₂
http://en.wikipedia.org/wiki/Dinitrogen_pentoxide

Apparently the oxidation state of Nitrogen is V.  Why is that?  It has five electrons circling it, one set is paired.  One of the electrons bonds to the oxygen in the middle of the molecule forming a single bond.  (now technically there is resonance between the other two but let's ignore it).  Another electron goes to a single bond with one of the oxygen atoms on the end.
Ok, so nitrogen has 3 electrons left circling it, 2 are paired, 1 is unpaired.  The final bond to the oxygen is a double bond.  The sigma bond grabs the first unpaired electron.  The pi bond will grab another electron, unless it grabs both paired nitrogen electrons, but that would be a problem because then there would be 3 electrons (2 from Nitrogen 1 from Oxygen) in the pi bond, as well as 2 in the sigma.  Can someone explain this to me please? 
Also, the oxygen in the middle would have a -2 oxidation state, while the others would technically be -1.5, correct?

[azmanam]

short answer: essentially 4 O^2- ligands for net -8 charge.  1 mu* O^2- ligand (we count this as 1 (-1) charge on each central atom) for total -10 charge of ligands.  Net neutral molecule.  Each nitrogen is +5.

http://bama.ua.edu/~kshaughn/ch609/notes/2-18-electron.pdf

*mu is inorganic for bridging.

[minimal]

short answer: essentially 4 O^2- ligands for net -8 charge.  1 mu* O^2- ligand (we count this as 1 (-1) charge on each central atom) for total -10 charge of ligands.  Net neutral molecule.  Each nitrogen is +5.

http://bama.ua.edu/~kshaughn/ch609/notes/2-18-electron.pdf

*mu is inorganic for bridging.

Ok I read that and I still really don't completely understand why nitrogen is V.  There are differences between covalent and ionic understandings, so basically the reason that N in ammonia is -III and N in ammonium is -III is the fact that there are just electrons that remain 'unaccounted' for?  Where does it go?

edit: basically I understand the rules that generally O is -2, except elemental and peroxides.  But there's no way that reality corresponds to that, it must form a more fundamental law.  Those just describe particular instances.  What does not describe particular instances is the rule that the oxidation state is the number of initial electrons - the electrons it would have if all bonds were completely ionic.  That is the method I am using that is leaving me blank here. 

[Borek]

I have not read your posts thoroghly, but could be you are trying to put too much weight into simplified theories. They are simplified, so they don't work always.

Especially oxidation numbers don't reflect any real physical property of atoms. They are just an accounting device that helps in balancing redox reactions, that's all.