Grammar

Obstacle is a noun referring to something that blocks or hinders progress. It names a thing, person, or condition that stands in the way of achieving something.

Obstetrics is a noun referring to the branch of medicine that deals with childbirth and the care of women giving birth. It names a specialized medical field or profession.

Obsolete is an adjective meaning no longer produced or used, out of date. It describes nouns (like obsolete technology or obsolete methods) rather than naming them or acting on them.

Obstinate is an adjective meaning stubbornly refusing to change one's opinion or course of action. It describes a person's character trait or attitude, not the person themselves or an action.

Obstruct is a verb meaning to block or hinder passage, movement, or action. It expresses an action that someone or something does to create an obstacle.

Oblivion is a noun referring to the state of being unaware or unconscious of what is happening, or the state of being completely forgotten or destroyed. It names a state or condition.

Oasis is a noun referring to a fertile spot in a desert where water is found. It names a place or thing, not an action or description.

Obdurate is an adjective meaning stubbornly refusing to change one's opinion or course of action. It describes a person's character quality, similar to obstinate.

Obtain is a verb meaning to get, acquire, or gain possession of something. It expresses the action of getting something rather than naming it or describing its qualities.

Obedience is a noun referring to compliance with the commands or orders of a person in authority. It names the quality or state of being obedient.

Oath is a noun referring to a solemn promise, often invoking a divine witness, regarding one's future action or behavior. It names a type of declaration or promise.

Oblige is a verb meaning to require or compel someone to take a certain course of action, or to do something as a favor. It expresses the action of forcing or helping.

Obligation is a noun - it names a state or condition of being bound to do something. Verbs express actions (like 'oblige'), adjectives describe qualities (like 'obligatory'), and adverbs modify how things happen (like 'obligatorily'). Prepositions show relationships between words, which doesn't fit here.

'Noise' is a noun that refers to any loud, unpleasant, or unwanted sound. Nouns name people, places, things, or ideas. 'Noise' fits this definition perfectly as it names a type of sound. It cannot function as a verb, adjective, adverb, or preposition in this form.

'Nominal' means relating to names or nouns, or being something in name only. Adjectives describe or modify nouns by providing more information about them. 'Nominal' functions as an adjective when describing something as nominal (e.g., 'nominal fee,' 'nominal role'). It doesn't function as a verb, pronoun, adverb, or preposition.