OS II/4 and AB II/5 under STCW: What Is the Difference

OS II/4 and AB II/5 under STCW: What Is the Difference

In maritime employment practice, confusion often arises between the positions of OS and AB, as well as between STCW II/4 and STCW II/5 certificates.
A lack of clear understanding of these STCW regulations may lead to contractual discrepancies, non-compliance with Flag State requirements, and remarks during Port State Control (PSC) inspections.This article provides a professional explanation of the differences between OS II/4 and AB II/5 strictly from the perspective of the STCW Convention.

STCW Structure and the Meaning of Chapter II

The STCW Convention (International Convention on Standards of Training, Certification and Watchkeeping for Seafarers) is structured into chapters, each regulating a specific shipboard department.STCW Chapter II — Master and Deck Department establishes the requirements for the training, certification, and functions of deck officers and deck ratings.Within Chapter II, the following regulations apply to deck ratings:

• Regulation II/4 — Ratings forming part of a navigational watch

• Regulation II/5 — Able Seafarer Deck

Accordingly, II/4 and II/5 represent different regulatory competency levels under STCW, not merely different job titles.

OS II/4 — Rating Forming Part of a Navigational Watch

OS II/4 is a deck rating holding a Certificate of Proficiency issued in accordance with STCW Regulation II/4.

STCW Status of OS II/4

• Level: Support level

• Function: Forming part of a navigational watch

• Certificate: Certificate of Proficiency — II/4

OS II/4 represents an entry-level qualification within the deck department.

Duties of OS II/4

• performing deck duties under direct supervision

• assisting in mooring and anchoring operations

• supporting navigational watchkeeping

• maintaining cleanliness and basic condition of deck areas

Responsibility of OS II/4

OS II/4:

• does not hold independent navigational responsibility

• performs duties strictly under instruction from AB II/5 or a deck officer

• is not authorised to make decisions affecting vessel safety

AB II/5 — Able Seafarer Deck

AB II/5 is a qualified deck rating certified in accordance with STCW Regulation II/5.

STCW Status of AB II/5

• Level: Support level (advanced)

• Function: Able Seafarer Deck

• Certificate: Certificate of Proficiency — II/5

AB II/5 is recognised by all Flag Administrations as a fully qualified working deck rating.

Duties of AB II/5

• independently standing navigational deck watch

• steering the vessel

• maintaining a proper lookout by sight and hearing

• participating in cargo operations

• supervising and guiding OS II/4 ratings

• performing safety-related deck operations

Responsibility of AB II/5

AB II/5:

• bears personal responsibility for the proper performance of assigned duties

• must identify hazards and immediately report them to the officer of the watch

• operates independently within the scope of certified competence

Key Differences Between OS II/4 and AB II/5

Is It Permissible to Work as AB with an OS II/4 Certificate

From the perspective of STCW, MLC 2006 and Flag State requirements — no.

• OS II/4 does not authorise employment in an AB II/5 position

• the position stated in the employment contract must correspond to the STCW regulation

• during PSC inspections, the STCW regulation number is decisive, not internal job titles

Any discrepancy may be considered non-compliance with STCW requirements.

Practical Importance for Seafarers

Understanding the distinction between OS II/4 and AB II/5 is essential when:

• signing an employment contract

• applying for vacancies

• planning career progression

• undergoing Flag State and PSC inspections

Correct certification is not only a matter of salary, but also of legal and professional security.

Conclusion

OS II/4 and AB II/5 are distinct regulatory levels under STCW, with different duties, responsibilities, and scope of authority.
Career progression within the deck department is possible only through formal transition from II/4 to II/5 in full compliance with STCW requirements.

Disclaimer

This article is for informational purposes only and is based on publicly available IMO and Flag State guidance. It does not constitute official flag state instruction or legal advice.