Bread will naturally dry out and become stale within about 24 hours. This
time does depend upon the relative humidity and temperature of the environment.
Natural yeasts and moulds will also start to colonise the bread from airborne
spores if a loaf is left uncovered. Fresh bread can become mouldy within a
couple of days or less.
Oil will help to retain moisture within the bread,
extending its shelf life. It also helps to make the dough more pliable and
easier to kneed.
An application of oil on the surface of the dough when it is rising will help
to retain the maximum amount of gas, giving a better rise, and prevent the dough
from sticking to the bowl.