онај ко / оно што

In Serbian, complex sentences often pair a demonstrative antecedent with a relative pronoun to form restrictive relative clauses.

1. Non-Individuated vs. Identified Referents (онај ко vs. онај који)
• Онај ко is used whenever the referent is non-individuated or unidentified by the speaker. This encompasses generic truths, rules, and proverbs (e.g., Онај ко ради, не боји се глади — 'He who works does not fear hunger'), as well as actual, concrete occurrences where the specific actor remains unknown or unspecified (e.g., Онај ко је синоћ разбио прозор мораће да плати — 'Whoever broke the window last night will have to pay'). However, it cannot refer to an already identified, concrete individual (*То је онај ко је јуче дошао is ungrammatical).
• By contrast, онај који specifies an identified or individuated person (e.g., То је онај човек који је јуче дошао), though it can also appear in general characterizations.

2. Number and Gender Restrictions
• The pronoun ко is invariable for gender and number; it is grammatically masculine singular. Predicates inside a relative clause governed by ко must always take masculine singular agreement (e.g., онај ко је дошао), regardless of the conceptual identity of the referent.
• Because ко cannot be feminine or plural, you cannot combine it with feminine or plural demonstratives (*она ко, *они ко).
• For feminine referents, use она која.
• For plurals, use они који (masculine/mixed), оне које (feminine), or она која (neuter).

3. Independent Case Assignment and Oblique Forms
Never force the relative pronoun to match the demonstrative's case (case attraction). Each clause assigns its own case independently based on its internal syntax:

• Помози ономе [dative] ко [nominative] нема никога. ('Help the one who has nobody.')
• Ово је за оног [accusative (animate form)] коме [dative] верујеш. ('This is for the one whom you trust.')
• Урадићу оно [accusative] о чему [locative] причаш. ('I will do what you are talking about.')

Standard Serbian requires оно што in relative structures and prescriptively rejects colloquial *оно шта. The standard neuter relative pronoun што shares its oblique declension paradigm with шта: genitive чега, dative/locative чему, and instrumental чиме (e.g., о чему; forms like *о што are ungrammatical).

4. Оно што vs. То што
Be careful to distinguish substantive content from factive clauses:
• Оно што refers to a thing, object, or statement: Разумем оно што кажеш. ('I understand what you are saying.')
• То што introduces a factual proposition ('the fact that'): Смета ми то што касниш. ('It bothers me that you are late.')

5. Headless Relatives and Obligatory Antecedents
Headless relatives (free relatives) do not require an overt demonstrative antecedent if the matrix clause and the relative clause assign matching cases—whether nominative (Ко пита, не скита) or accusative (Узми шта хоћеш; Позови кога желиш). However, an overt demonstrative antecedent becomes syntactically mandatory when there is a case mismatch between clauses, or when a preposition is required in the main clause, because prepositions cannot directly govern a bare relative pronoun (e.g., Разговарао сам са оним ко води пројекат, never *Разговарао сам са ко води пројекат).