How I would think of it.
How many degrees of unsaturation? Zero (O.Chem knowledge)
This means that there are no pi bonds or rings so the oxygen can only contribute by way of an alcohol or ether.
With an alcohol, there are 4 carbons so 4 different locations for the alcohol to form.
There are 3 locations for an ether to form (not 4 because the 4th would be a terminal carbon).
This gives you 7.
Or you can draw all of the possible constitutional isomers.