A oxigenação melhora cada vez mais.

A is the feminine singular definite article, equivalent to the.

It is used because oxigenação is a feminine singular noun:

• a oxigenação
• a melhora
• a recuperação

In Portuguese, it is very common to use the definite article with nouns even when English might not always use the.

In Portuguese, every noun has a grammatical gender. Oxigenação is feminine because nouns ending in -ção are almost always feminine.

Examples:

• a situação
• a informação
• a respiração
• a oxigenação

So the article must be feminine too: a oxigenação.

Here, melhora is a verb, from melhorar (to improve).

So:

• A oxigenação melhora = The oxygenation improves / is improving

This can confuse learners because melhora can also be a noun in other contexts:

• Houve uma melhora. = There was an improvement.

But in your sentence, it is definitely the verb, because it follows the subject A oxigenação.

Melhora is the present indicative form:

• A oxigenação melhora... = The oxygenation improves / is improving...

Portuguese often uses the simple present where English might prefer is improving.

If you said:

• A oxigenação está melhorando

that would also be correct, and it puts more emphasis on the ongoing process, like is getting better or is improving right now.

So both are possible, but:

• melhora = simple, general, natural
• está melhorando = more explicitly progressive

Cada vez mais means more and more.

Literally, it is something like:

• cada vez = each time
• mais = more

Together, the expression means that something is increasing progressively over time.

Examples:

• Ela está cada vez mais confiante. = She is getting more and more confident.
• O problema fica cada vez mais sério. = The problem gets more and more serious.

So:

• A oxigenação melhora cada vez mais. = The oxygenation improves more and more.

Because cada vez mais expresses a continuing increase over time, not just a simple comparison.

Compare:

• A oxigenação melhora mais. = The oxygenation improves more.
  This sounds incomplete unless you compare it with something else.
• A oxigenação melhora cada vez mais. = The oxygenation improves more and more.
  This clearly means the improvement keeps increasing.

So cada vez mais is the natural phrase for more and more.

You might see article omission in headlines, notes, or very technical writing, but in normal standard Portuguese, A oxigenação melhora cada vez mais sounds more natural and complete.

Portuguese generally likes using articles before nouns much more than English does.

So for a learner, the safest and most natural choice is:

• A oxigenação melhora cada vez mais.

Yes, absolutely. That is a very natural alternative.

Compare the two:

• A oxigenação melhora cada vez mais.
  Focuses on the process of improving.

• A oxigenação está cada vez melhor.
  Focuses more on the result/state: it is better and better.

Both are correct, and in many contexts they are very close in meaning.

Yes. This pattern is very common:

[subject] + [verb] + cada vez mais

Examples:

• Ele trabalha cada vez mais. = He works more and more.
• A cidade cresce cada vez mais. = The city grows more and more.
• Os preços aumentam cada vez mais. = Prices increase more and more.

So A oxigenação melhora cada vez mais follows a very normal Portuguese pattern.

In Brazilian Portuguese, oxigenação is pronounced approximately like:

o-xi-ge-na-ÇÃO

The stress falls on the last syllable because of the ending -ção.

A rough English approximation is:

• oh-shee-zheh-nah-SOWN

A few pronunciation notes:

• x in oxi- is usually pronounced like sh in Brazilian Portuguese
• ge sounds like the s in measure or zh
• ção is a nasal sound, very common in Portuguese

The noun oxigenação comes from oxigenar (to oxygenate) and is related to oxigênio (oxygen).

So the word family is:

• oxigênio = oxygen
• oxigenar = to oxygenate
• oxigenação = oxygenation

This is useful because many Portuguese medical or scientific words are built in similar ways.