IEC 60079-6:2015 specifies the requirements for the design, construction, testing and marking of Ex Equipment and Ex Components with type of protection liquid immersion "o" intended for use in explosive gas atmospheres. Ex Equipment and Ex Components of type of protection liquid immersion "o" are either: - Level of Protection "ob" (EPL "Mb" or "Gb") or - Level of Protection "oc" (EPL "Gc"). For Level of Protection "ob", this standard applies where the rated voltage does not exceed 11 kV r.m.s. a.c. or d.c. For Level of Protection "oc", this standard applies where the rated voltage does not exceed 15 kV r.m.s. a.c. or d.c. This standard supplements and modifies the general requirements of IEC 60079-0. Where a requirement of this standard conflicts with a requirement of IEC 60079-0, the requirement of this standard takes precedence. This fourth edition cancels and replaces the third edition, published in 2007. This edition constitutes a technical revision. The significant changes with respect to the previous edition are: - Edition 4 represents a major technical revision of the requirements for oil immersion "o" and should be considered as introducing all new requirements. The normal "Table of Significant Changes" has not been included for this reason. In particular: - The requirements for oil immersion "o" have been redefined into liquid immersion, levels of protection "ob" and "oc" as recommended by the responses to 31/715/DC; - The ability to protect sparking contacts has been added to both "ob" and "oc". Additional requirements have been introduced for the protective liquid. Keywords: Ex Equipment and Ex Components with type of protection liquid immersion "o"