Draw the Orbital Overlap Diagram of O2 (Oxygen gas) 

O2 is usually considered to be DOUBLE bonded.
This means each oxygen atom is sp2 hybridized; I draw the electron configuration diagram in this video.
Then, I show you how to draw the sp2 hybrid orbitals (in a trigonal planar arrangement) and the unhybridized, leftover, 2p orbital which is above and below the bond axis. These unhybridized orbitals overlap side-to-side to form the pi bond.
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