This is the
g* molecular orbital. It is the highest occupied MO. There are two of them; one formed from the antisymmetric combination of px orbital and the other from the py orbital. In oxygen there is one electron in each orbital so the resulting state is a triplet (S=1) and dioxygen is paramagnetic. Note there is a node between the nuclei which makes it antibonding and a node along the z axis which makes it 
This is the
u molecular orbital. Like the * orbitals there are two of them. One is formed by the symmetric combination of px orbital and the other from the py orbital. Note there is no node between the nuclei so it is bonding but there is a node parallel to the z axis which makes it . 
This is the
g* molecular orbital. It is the highest occupied MO. There is one of them. It is formed by the antisymmetric combination of py atomic orbitals. Note that the nodes are at the nuclei but there is no node between the nuclei nor along the z axis. That make it bonding.
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