You oblique pronouns o(s) and a(s) they also occupy the function of demonstrative pronouns, always functioning as noun pronouns. Any questions about what “noun pronouns” are? If so, check out some clarification on this subject, below:
The noun pronouns are those that replace the noun, thus avoiding an idea that would perhaps become repeated. Thus, as substantive pronouns, the pronouns o (s) and a (s) are used in the cases expressed below:
* In cases where they appear determined by a prayer or, in rare cases, by an adjective expression, denoting the meaning of “that (s), that (s) and that”. Let's look at an example:
The good student is responsible. O who does not fulfill his obligations is truly harmed.
We found that the pronoun that is highlighted, besides being able to be replaced by “that one”, still has a substantive function, replacing the word “student” – in turn, a noun.
I can not find The that I kept waiting.
Identical to the previous analysis, given that the term “person”, “girl”, among others, are implied.
* Another case refers to the condition of such pronouns, which can be replaced by "this, this and that", exercising the functions direct object or predicative syntactics, referring to a noun, adjective, the general sense of a sentence or a term of it. Let us verify:
Don't think it wasn't kind, because O was.
The answer you were going to give, you knew-The beforehand.
By Vânia Duarte
Graduated in Letters
Source: Brazil School - https://brasilescola.uol.com.br/gramatica/pronomes-s-s-como-demonstrativos.htm