The noun 'oxygen' is an uncountable (mass) noun, a word for something that is indivisible into countable units.
Units of uncountable nouns are expressed by adjectives or a partitive noun (also called a noun counter).
The noun 'oxygen' takes a verb for singular unless quantified in the plural; for example:
Singular
Singular Plural is caterpillars
The word crisis is singular. The plural form of crisis is crises.
Steroids is plural. The singular form of the word is steroid.
Association is singular. Associations is plural.
The word team is singular; the plural form is teams.
These is plural, this is singular
Lance is singular. Lances is plural.
Singular
The word 'Phenomenon' is singular. The plural form is 'Phenomena'.
The word metastasis is singular, metastases is plural.
The word "ground" can be both singular and plural. Singular: "The ground is wet." Plural: "The grounds of the park are well-maintained."
Louse is singular. The plural form is lice.
No, the word "list" is singular. The plural form of "list" is "lists."
Verbs cannot be singular or plural. Has is after singular nouns.
It's singular. The plural is cafeterias.
Tool is singular. Tools is plural.