Oxalate Anion MO Diagram
Here's how I would approach it. I would start by making some (over)simplifying assumptions:

1) O $p_x$ orbitals are mostly nonbonding, so I'm going to ignore interactions of those orbitals with carbon orbitals

2) Due to $sp$ mixing, the carbon $p_y$ and $s$ orbitals can be merged into $sp$ orbitals aligned on the $y$ axis.

3) Same for the O $p_y$ and $s$ orbitals. In this case, the mixed $sp$ orbital with its large lobe oriented away from C is mostly nonbonding, so I'll ignore interactions of it with carbon orbitals

With those assumptions in place, I focus on building and ordering the occupied molecular orbitals. The highest energy occupied orbitals will be some of those oxygen-based nonbonding orbitals, so let's start by putting all eight of them at the top of the diagram. The relative energy levels among them will be affected by the number of positive and negative interactions, so the highest energy one will likely be the $b_{1g}$ where all of the $p_x$ orbitals have anti-bonding interactions with their neighbors. I mention this orbital in particular because it is the HOMO, and I think that the $\sigma$ interaction between oxalate and a metal center would come from the interaction of this orbital with the metal $d_{x^2-y^2}$ or $d_{xy}$, for example. 

Mixed in with the $p_x$ and $sp_y$ nonbonding orbitals will be the two $\pi_{nb}$ orbitals, also centered on the oxygens. These are the $b_{2g}$ and $a_u$ orbitals in your set above. Since there isn't a carbon $\pi$ orbital with the right symmetry to interact with either of these, the resulting MO's have a nodal plane aligned with the C-C bond. These are the orbitals that will have $\pi$ interactions with a metal center if the oxalate is acting as bidentate ligand.

Below the nonbonding $\pi$ orbitals, we have the two $\pi$ bonding orbitals formed from the $b_{1u}$ and $b_{3g}$ central and oxygen $p_z$ orbital combinations. The $b_{3g}$ is higher energy of the two because of the C=C $\pi^*$ interaction.

At the bottom of the occupied orbitals, we have the five primarily $\sigma$ orbitals. I think the highest energy of the group is the $\sigma_{C-C}$ formed from the mixed $sp_y$ carbon orbitals oriented towards each other, ie a mix of the two $a_g$ central orbitals. Since the density is primarily between the carbons, I'm going to ignore the interactions with the O orbitals.

The four $\sigma_{C-O}$ orbitals come from the pairings of the central $b_{2u}$, $b_{1g}$, $b_{3u}$ and $a_g$ orbitals with their ligand orbital counterparts. In energy, from low to high I would guess $a_g$ ($\sigma_{C-O}$ and $\sigma_{C-C}$), $b_{3u}$ ($\sigma_{C-O}$ and $\pi_{C-C}$), $b_{1g}$ ($\sigma_{C-O}$ and $\pi^*_{C-C}$), then $b_{2u}$ ($\sigma_{C-O}$ and $\sigma^*_{C-C}$).