It is very common, and in fact recommended, to leave some heating oil in your tank during warmer months. If you've used heating oil only for one year so far, though, just what to do now that another winter is here may be a little unclear. The oil from last year should be good to use, but you still need to get new oil for this season, and you may seriously want to look into having the oil tank cleaned as well.
You've Got a Couple of Years in General
Heating oil does not go bad quickly; in general, the heating oil you put in the tank now can stay good to use for about a couple of years. But you do want to use the oil up, and you don't want to assume that since the oil has been of good quality for a couple of years, you can stretch it out a third year. Say you have to work in another city for a few months and close up your house, so you don't use the heating oil you have. It should be okay to use the next year, but you really do want to finish it up by then.
The Tank's Interior Condition Plays a Role
If the interior of your heating oil tank is in great shape, then the oil inside can stay good for a long time when you aren't using up the oil. But dirt, dust, and other contamination can affect the quality of the oil, which means that if you know dirt has gotten inside, or you have not had the inside of the tank cleaned for many years, you need to contact the company that sold or rented the tank to you and ask them about cleaning. Don't try it yourself, especially if there is a warranty for the tank or a maintenance agreement in the rental contract.
You Want the Tank to Always Have Some Oil in It (For the Most Part)
It's good that you had some heating oil left over from last year because your home heating oil tank is similar to your car's gas tank. The emptier it is, the bigger the chance of condensation forming in the rest of the tank, which isn't optimal. By keeping the tank at least, say, half full, you reduce the chances of condensation forming. Plus, that half-tank lets you heat your home if there's an early cold snap and you haven't gotten your first delivery of the season yet. One thing you should not do is assume that since you have all that oil you can wait to get your next delivery; again, you want that tank to stay as full as possible to avoid condensation inside. The only exceptions are when you know you have old oil to use up that's reaching the end of its optimal quality, and when the tank is being cleaned.
If you've been using up that store of older oil to heat your home, you need to call now to have the tank filled. Remember that as the weather gets colder, the demand for home heating oil goes up, and that means prices can go up as well. If you want to spend less, call before it gets much colder.
Reach out to a heating oil company to learn more.