In short

Greek has two relative pronouns. One, που, never changes shape and does all the everyday work (lesson 37). The other, ο οποίος, is a full nominal: it carries an article, and it inflects for gender, number and case like an adjective in -ος, -α, -ο. Because it inflects, it says out loud what που leaves to guesswork — which noun it points back to, and what job it does inside its own clause. That precision costs register: ο οποίος belongs to newspapers, contracts, court decisions and academic prose, and sounds starched in ordinary conversation.

The whole lesson hangs on one split: gender and number come from the antecedent, case comes from the role inside the relative clause. Get those two sources of information from two different places and the form writes itself.

The rule

1. It is article + pronoun, always together. The unit is ο οποίος, η οποία, το οποίο — never a bare οποίος. The article inflects with the pronoun, and after a preposition the preposition governs the article: με την οποία, σε → στους οποίους, από τα οποία.

2. Gender and number: look left, at the antecedent. Η απόφαση is feminine singular, so the pronoun is feminine singular no matter what it does next: η απόφαση η οποία ελήφθη, η απόφαση την οποία πήραμε, η απόφαση για την οποία συζητήσαμε.

3. Case: look right, inside the clause. The pronoun takes the case its own verb or preposition demands. Subject → nominative. Direct object → accusative. Possessor → genitive. Object of a preposition → whatever that preposition takes (in demotic, the accusative; with learned prepositions, the genitive — lesson 63). The antecedent’s case is irrelevant: in Ευχαρίστησα τον γιατρό ο οποίος με χειρούργησε, τον γιατρό is accusative but the pronoun is subject of χειρούργησε, so it is nominative.

4. This is exactly where που fails. Που cannot be inflected and cannot follow a preposition, so when the relativized slot is anything other than subject or direct object, spoken Greek either leaves the relation unstated (ο λόγος που έφυγε, το κατάστημα που ψωνίζω, η πόλη που ζω) or plugs the hole with a resumptive weak pronoun later in the clause — and for an indirect object or a possessor that pronoun is not optional:

• ο άντρας που του μίλησα (“the man that I spoke to him”) → written: ο άντρας στον οποίο μίλησα
• το θέμα που μιλήσαμε γι’ αυτό → written: το θέμα για το οποίο μιλήσαμε
• ο συγγραφέας που διάβασα το βιβλίο του → written: ο συγγραφέας του οποίου το βιβλίο διάβασα

The two strategies are mutually exclusive. Ο οποίος already carries the case, so a resumptive clitic on top of it is ungrammatical.

5. Disambiguation. With two candidate antecedents, που points at both and ο οποίος points at one: η κόρη του υπουργού, η οποία μιλάει τρεις γλώσσες (the daughter) against η κόρη του υπουργού, ο οποίος μιλάει τρεις γλώσσες (the minister). Non-restrictive clauses — the ones set off by a comma — also read better with ο οποίος in writing.

6. Register. Use ο οποίος in reports, essays, official letters, press and legal texts, or when clarity genuinely requires it. In speech, prefer που; a chatty sentence built on ο οποίος sounds like a bureaucrat reading aloud. Free relatives with no antecedent at all (“whoever”, “whatever”) are a separate construction — lesson 46.

Forms & patterns

Singular. Note that the accent never moves: it sits on -οί- in every cell (οποίος, οποίου, οποίων), unlike a noun such as ο άνθρωπος → του ανθρώπου.

Plural:

A relative pronoun has no vocative — you cannot address it — so those cells are permanently empty. The masculine accusative singular is τον οποίο in ordinary written Greek; the fuller τον οποίον — and, in older legal prose, even την οποίαν — is a katharevousa retention on the pronoun itself, written before any sound, so it is not an instance of the phonological final -ν rule of lesson 8 (that rule governs only τον, την, έναν, δεν, μην, αυτόν).

After a preposition the article does the visible work:

The same relation, said twice — spoken strategy against written strategy:

Examples

Common mistakes

• Ευχαρίστησα τον γιατρό τον οποίο με χειρούργησε. -> Ευχαρίστησα τον γιατρό ο οποίος με χειρούργησε. — the case is copied from the antecedent instead of being taken from the role inside the clause; there the pronoun is the subject of χειρούργησε.
• Η πρόταση η οποία κατέθεσε η επιτροπή απορρίφθηκε. -> Η πρόταση την οποία κατέθεσε η επιτροπή απορρίφθηκε. — the pronoun is the object of κατέθεσε (η επιτροπή is the subject), so it must be accusative even though η πρόταση is nominative.
• Η απόφαση το οποίο πάρθηκε χθες… -> Η απόφαση η οποία πάρθηκε χθες… — gender and number are never chosen freely: they come from the antecedent η απόφαση.
• το θέμα οποίο συζητήσαμε -> το θέμα το οποίο συζητήσαμε — the article is part of the pronoun and can never be dropped.
• η συνάδελφος με οποία συνεργάζομαι -> η συνάδελφος με την οποία συνεργάζομαι — after a preposition the article is still obligatory, and it is the article that shows the case.
• ο συνάδελφος στον οποίο του έδωσα τον φάκελο -> ο συνάδελφος στον οποίο έδωσα τον φάκελο — ο οποίος already encodes the indirect object; the resumptive clitic belongs to the που strategy only.
• το θέμα το οποίο μιλήσαμε -> το θέμα για το οποίο μιλήσαμε — you cannot delete the preposition; μιλάω needs για here, and ο οποίος is precisely the form that lets it surface.
• ο άντρας στον οποίο μίλησα, ρε παιδί μου -> ο άντρας που του μίλησα — grammatically fine but tonally wrong; in casual speech use που with its clitic.

Tips

• Two questions, two directions. Look left for gender and number (what noun is this about), look right for case (what job does it do here). Never let one answer both.
• Test for case by rebuilding the clause with a plain noun: κατέθεσε την πρόταση → accusative → την οποία. It works every time.
• If a preposition is involved, ο οποίος is the only clean option. Που either leaves the relation to be guessed from context or picks it up with σ’ αυτόν, γι’ αυτό, μαζί της later in the clause — acceptable when spoken, untidy when written.
• Never combine the two systems: preposition or genitive on ο οποίος means no weak pronoun afterwards; που means the weak pronoun is required.
• For “whose”, the usual order puts the genitive first and the possessed noun with its article right after it: ο συγγραφέας του οποίου το βιβλίο διάβασα. The reverse — ο συγγραφέας το βιβλίο του οποίου διάβασα — is also correct, just rarer; both are formal.
• Use it deliberately, not as decoration. One ο οποίος that resolves an ambiguity reads as precise; three in a paragraph read as a translated legal notice.
• After a comma — a non-restrictive, parenthetical clause in a written text — ο οποίος is the natural choice, and the comma is what tells the reader the clause adds information rather than narrowing it.