When buying home heating oil you can usually get a better deal when you pre buy the heating oil during the summer months when it's in less demand.
During the winter months and even more toward the end of the year the heating oil prices tend to spike since the heating oil will be in more demand which drives up the price of heating oil.
It's a good idea to pre-buy the home heating oil early but it's not a good idea to lock in a price for the heating oil each month or year since the heating oils price could go down during that time which means that you could end up paying more per gallon of heating oil.
What I would do is just wait until summer rolls around to have your heating oil tank refilled after the last tank fill during the winter for the best price.
Of course you do need to have your heating oil tank refilled during the winter months but after your last tank of heating oil is used up for the winter then wait until the summer to get it completely refilled for the next winter to save money on the heating oil bill.