Mitsui O.S. Lines, Nippon Yusen Line and Kawasaki Kisen Line are known as Japan's three shipping companies. They rank first in Japan in terms of net profit and total market value, and their sales are second only to Nippon Yusen Line. Mitsui O.S. Lines has two main origins: Osaka Merchant Line, founded in 1884, and Mitsui Shipbuilding & Marine Engineering Co., Ltd. in 1942, which belong to Japan's two major conglomerates, Sumitomo Zaibatsu and Mitsui Zaibatsu. In 1964, Osaka Merchant Line and Mitsui Shipbuilding & Marine Engineering merged to form Osaka Mercantile Line Mitsui Marine Co., Ltd., setting a precedent for the merger of major cross-zaibatsu companies in Japan. In April 1999, Mitsui O.S. Lines merged with Navix Line, Japan's fourth largest shipping company at the time, and was renamed Mitsui O.S. Lines.

Unlike Maersk and other shipping companies that focus on container transportation, MOL's business scope involves various ship types. As of the end of March 2008, the country had a total of 845 merchant ships of various types with a weight of 54.34 million tons, making it the world's largest fleet. At the same time, Mitsui Lines has the industry's largest LNG fleet, as well as powerful dry bulk carriers and car roll-off ships, and ranks around 10th in the world in container transportation. Mitsui Lines' containers are symbolized by a crocodile carrying the container, which is an obvious and interesting feature.

In the 2008 Fortune Magazine ranking of the world's top 500 companies, Mitsui Lines, Maersk, Nippon Yusen Lines and COSCO were the only four international shipping companies selected.