El oído necesita descanso.

In Spanish, it is very common to use the definite article with body parts and with nouns used in a general sense.

So el oído here means something like the ear / hearing as a general concept, not a specific ear. Spanish sounds more natural with the article here than English does.

It can mean either, depending on context.

• el oído can refer to the ear as an organ
• it can also refer to hearing or the sense of hearing

In El oído necesita descanso, it often feels a bit broader than just the physical ear. It can suggest that your hearing / ears need rest.

The accent mark shows that the í is pronounced separately from the o.

So oído is pronounced in three syllables:

o-í-do

Without the accent, Spanish spelling rules might make you read the oi more like a single unit. The accent forces a break, called a hiatus.

It is pronounced roughly like:

oh-EE-doh

The stress falls on í:

o-Í-do

Make sure not to squash the first two vowels together too much. It should sound like three clear syllables.

Necesita is the third-person singular present form of necesitar (to need).

The subject is el oído, which is singular, so the verb must also be singular:

• el oído necesita
• los oídos necesitan

So necesita matches el oído.

Because the subject is already stated: el oído.

Spanish does not add an extra subject pronoun unless it is needed for emphasis or contrast. So:

• El oído necesita descanso = normal
• Él would mean he, so it would not fit here anyway

In this sentence, el is the article the, not the pronoun he.

Both are possible, but they are slightly different.

• necesita descanso = needs rest
• necesita un descanso = needs a break / needs a rest

Without un, it sounds more general and abstract. It refers to rest as a thing that is needed, not necessarily one specific break.

That is why necesita descanso works very naturally here.

Yes. That would also be grammatical and natural.

The difference is:

• necesita descanso = needs rest
• necesita descansar = needs to rest

The version with descanso uses a noun. The version with descansar uses an infinitive verb. Both are correct; the original sentence is just a little more compact and formal-sounding.

Because oreja and oído are not exactly the same.

• oreja usually means the outer ear
• oído can refer to the ear more generally, especially in relation to hearing

So if you are talking about hearing, listening fatigue, loud noise, or ear health in a broader sense, oído is often the better word.

Yes, it is natural, especially in a general, medical, or informative style.

It sounds a bit like something you might read in:

• a health article
• a warning about noise
• advice about hearing care

In everyday speech, people might also say things like:

• Tus oídos necesitan descanso.
• El oído necesita reposo.
• Tus oídos necesitan descansar.

But El oído necesita descanso is perfectly good Spanish.