Oxygen in water: How is it single if it's always double?
I think you are confusing something here: Molecular oxygen is most commonly found as the gas we know, $\ce{O2}$ and ozone, $\ce{O3}$. However, this doesn't limit it to only that. Oxygen can appear in a wide variety of (bio-) chemicals and there, it only needs two bonds to other atoms (or a double bond to one atom).

Good examples for this are:


The aforementioned water $\ce{H2O}$ where the oxygen is bonded to two hydrogen atoms. 
One of my most loved molecules: Ethanol. Here, the oxygen is bonded to a hydrogen and a carbon atom.
In an ester, which usually are quite fragrant compounds, there actually are two oxygen atoms. But they are not connected to each other. Instead, one has a double bond and the other two bonds, each of them to carbon respectively.


I hope this has made it clear that oxygen doesn't have to bond to itself (but it can, see peroxides), and that oxygen doesn't have to come in pair of twos for the molecule to not fall apart.