Lone pairs in MO theory?
Short answer: 

Non-bonding MOs are usually lone pairs, but a lone pair does not necessarily mean there is a non-bonding MO.

Long answer:

You are trying to match what you learned from Lewis structures to the orbital picture. In general, that does not work (there may be exceptions).

Let's look at the MO diagram:



It is tempting to assign the bonding $\sigma_{2p}$ to the first (sigma) bond in $\rm O_2$. The two $\pi_{2p}$ bonding orbitals would then results in 2 $\pi$ bonds, hence we get a O-O triple bond. The further two electrons in the anti-bonding $\pi_{2p}^*$ orbitals could be assigned to one oxygen each, thus we have a biradical. The remaining two electron lone pairs might be assigned to the two $\sigma_{2s}$ orbitals. This is actually one of the two valid Lewis structures for $\rm O_2$.
(The other possible Lewis structure has only a double bond and two lone pairs at each oxygen atom.)

The problem is that the lone pairs are ascribed to only one atom, but the MOs are delocalized over the molecule. Thus there is no direct relationship between them. A non-bonding orbital is usually just an atomic orbital, which you may thus ascribe to a lone pair. But that does not mean each lone pair leads to a non-bonding orbital.

It is however possible, to localize the molecular orbitals (by making appropriate linear combinations of MOs) in such a way you get atomic orbitals, which you might match again with the lone pairs. The thing here is, that there are many different sets of orbitals which are all valid descriptions of the bonding situations.

Also note, that MO diagrams are just a simplification as well.

Source of the MO Diagram