The verb must agree with its subject in number and person. That is the verb must be of the same number and person as its subject. If the subject is plural, the verb must be in the plural. Similarly, if the subject is singular the verb must be singular.

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.