Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Call emergency services now if there is:
And for anyone, someone who:
Contact a poison information service promptly after fuel oil is swallowed, even if there are no symptoms.
Do not wait for coughing or breathing difficulty to appear.
In the United States, Poison Control is available at 1-800-222-1222 .
Elsewhere, use your national poison centre or local urgent medical service.
Seek urgent assessment for ongoing vomiting, chest pain, low oxygen readings, worsening drowsiness, or eye pain and blurred vision after rinsing.
A cough, fever, wheeze, or shortness of breath that appears or worsens in the hours after exposure also needs prompt review.
New breathing difficulty is not a routine follow-up issue and should be assessed urgently.
Routine follow-up is appropriate for a cough, reduced stamina, skin rash, or other symptoms that continue after initial treatment but are not rapidly worsening.
The article also says
Move into fresh air only if you can do so safely.
Do not enter an enclosed contaminated area to rescue someone without specialist training and protective equipment.
Every sign above is quoted from the sofidex article on Fuel oil poisoning. If you are not sure, err upward — it is always reasonable to be checked.