When you buy heating oil it's best to purchase a new supply of heating oil during the summer months since during the summer months the demand for heating oil will be much lower and as a result the cost for the heating oil during the summer months will be cheaper.
During the fall and winter months and especially during the end of the year demand for heating oil rises significantly which results in higher prices for the consumer of the heating oil.
So if you want a cheaper price then get your heating oil tank filled up during the summer to save some money.