Incorrect: Petrol or kerosene are used.
Correct: Petrol or kerosene is used.
Two singular nouns joined by or take a singular verb.
Incorrect: A melon or grapes is suitable.
Correct: A melon or grapes are suitable.
Correct: Grapes or a melon is suitable.
When a singular noun and a plural noun are joined by or, the verb agrees with the nearest noun – it would be better to use the plural noun second; then a plural verb would be used.
Incorrect: One of my students sing well.
Correct: One of my students sings well.
Incorrect: One of the eggs are bad.
Correct: One of the eggs is bad.
A singular noun should be used after one of and a plural noun.
Incorrect: Bullets or a bomb are not allowed on the plane.
Correct: Bullets or a bomb is not allowed on the plane.
Correct: A bomb or bullets are not allowed on the plane.
Incorrect: Hundred dollars are too much to lose.
Correct: Hundred dollars is too much to lose.
When the subject is a sum of money considered as a whole, the singular verb is used.