The Office of Legal Affairs (OLA) is the principal legal arm of the United Nations Secretariat, established in its modern configuration by Secretary-General Trygve Lie shortly after the Organization's founding and consolidated through successive Secretariat reorganizations, most notably the 1992 restructuring under Secretary-General Boutros Boutros-Ghali. Its mandate derives from Article 97 of the UN Charter, which empowers the Secretary-General to appoint staff under regulations established by the General Assembly, and from a succession of General Assembly resolutions assigning specific functions — including depositary responsibilities under Article 102 of the Charter (registration and publication of treaties) and secretariat support to the International Law Commission established by resolution 174(II) of 1947. The Office is headed by the Under-Secretary-General for Legal Affairs and United Nations Legal Counsel, a post that ranks among the most senior in the Secretariat and that speaks authoritatively on questions of international and institutional law on the Secretary-General's behalf.
Procedurally, OLA operates through six specialized divisions, each with distinct intake and clearance procedures. A request for legal advice from a UN organ, a peacekeeping mission, or a Member State is routed to the relevant division — the Office of the Legal Counsel for constitutional and institutional questions, the General Legal Division for contracts and claims, the Treaty Section for depositary actions, the Codification Division for international law development, the Division for Ocean Affairs and the Law of the Sea (DOALOS), or the International Trade Law Division (which services UNCITRAL). Opinions are drafted, peer-reviewed within the division, and where the matter touches the Organization's institutional interests, cleared at the level of the Legal Counsel before transmission. Formal opinions of the Legal Counsel are circulated as documents of the requesting organ and published in the United Nations Juridical Yearbook.
The Treaty Section performs the operational mechanics of UN depositary practice for over 560 multilateral instruments. When a state deposits an instrument of ratification, accession, or reservation, the Section verifies the credentials of the signatory under the Vienna Convention on the Law of Treaties (VCLT) Articles 7 and 77, examines reservations for prima facie conformity, circulates a Depositary Notification (the "C.N." series) to all states concerned, and registers the treaty under Charter Article 102 — a step without which the treaty may not be invoked before any UN organ. The Section also maintains the United Nations Treaty Series (UNTS) and the online United Nations Treaty Collection database.
Contemporary practice illustrates the breadth of OLA's work. The Office advised on the legal framework for the United Nations Verification Mission in Colombia following the 2016 peace accord; it negotiated the Status-of-Forces Agreements (SOFAs) governing MINUSMA in Mali before that mission's 2023 termination; it provided the legal basis for the Independent Investigative Mechanism for Myanmar created by Human Rights Council resolution 39/2 in 2018; and it has handled depositary functions for the Treaty on the Prohibition of Nuclear Weapons, which entered into force in January 2021. The current Legal Counsel, Elinor Hammarskjöld of Sweden, assumed office in 2025, succeeding Miguel de Serpa Soares of Portugal who served from 2013 to 2024.
OLA must be distinguished from several adjacent bodies with which it is frequently conflated. It is not the International Court of Justice, the UN's principal judicial organ seated at The Hague, though OLA represents the Secretary-General in proceedings before the ICJ, including advisory opinion requests. It is separate from the International Law Commission, an expert body of 34 jurists elected by the General Assembly whose secretariat services are provided by OLA's Codification Division but whose work product is independent. It also differs from the Office of Administration of Justice, which handles internal staff disputes through the UN Dispute Tribunal and Appeals Tribunal, and from the legal offices of specialized agencies such as the WHO Office of the Legal Counsel, which operate autonomously under their respective constitutions.
Controversies have periodically surrounded the confidentiality of OLA opinions. Member States have at times pressed for disclosure of advice given to the Secretary-General on contested questions — the legality of the 2003 Iraq intervention, the status of Kosovo following its 2008 declaration of independence, and the recognition of competing credentials (most recently the Myanmar and Afghanistan seats at the General Assembly's Credentials Committee). OLA's practice is to publish opinions of general legal interest while withholding privileged advice to the Secretary-General, a distinction that critics argue lacks transparency. The Office has also faced scrutiny over the immunities asserted under the 1946 Convention on the Privileges and Immunities of the United Nations, particularly in the Haiti cholera litigation (Georges v. United Nations, dismissed by the Second Circuit in 2016), where OLA invoked absolute immunity over claims arising from the 2010 outbreak.
For the working practitioner, OLA functions as both gatekeeper and resource. A treaty negotiator must coordinate with the Treaty Section on final clauses, authentication procedures, and the mechanics of signature ceremonies — failure to do so risks invalid deposits. A desk officer drafting a Security Council resolution will find OLA's Security Council Affairs colleagues (under DPPA, in close liaison with OLA) policing language for consistency with prior Chapter VII determinations. A mission legal adviser confronting a host-state dispute over privileges and immunities will route the matter through OLA for formal protest. Understanding OLA's divisional structure, clearance timelines, and the weight accorded to Legal Counsel opinions in subsequent practice is indispensable for anyone operating within the UN system.
Example
In January 2021, the UN Office of Legal Affairs Treaty Section issued Depositary Notification C.N.602.2020.TREATIES-XXVI.9 confirming that the Treaty on the Prohibition of Nuclear Weapons had reached fifty ratifications and would enter into force.