Dear can a landlord wait for ten months before sending a bill?
Whether your lease allows for the landlord's delay or does not allow for the delay, your landlord waited ten month to send you a bill. I take it that he did not break down the oil charges by delivery date, gallons and price per gallon, and did not provide you with an itemization for the gas (propane or municipal?)
Without an itemization you have no way to ascertain the accuracy of the landlord's invoice to you, so you could first force him to provide an itemization with copies of the slips from the oil deliveries, as well as for the propane or from his gas utility charges. He may not be able to back up the claim you owe $1500.00.
The fact that he delayed the passing on to you of costs you agreed to pay would not likely be prohibited by the lease if the lease has a standard "no waiver" provision. If you are in doubt about the terms of the lease consult with an attorney.
The answer provided to you is in the nature of general information. The general proposition being that you should try to avoid a bad outcome if you can.