Seawise Giant was ordered in 1974 and delivered in 1979 by Sumitomo Heavy Industries, Ltd. (S.H.I.) at her Oppama shipyard in Yokosuka, Kanagawa, Japan, as a 418,611-ton Ultra Large Crude Carrier (ULCC).[10] The vessel remained unnamed for a long time, and was identified by her hull number, 1016. During sea trials, 1016 exhibited severe vibration problems while going astern. The Greek owner refused to take delivery and the vessel was subject to a lengthy arbitration proceeding. Following settlement, the vessel was sold and named Oppama by S.H.I.

Size comparison of some of the longest ships. From top to bottom: Knock Nevis (ex-Seawise Giant), Maersk Mc-Kinney Møller, Vale Brasil, Allure of the Seas, and USS Enterprise (CVN-65).
The shipyard exercised its right to sell the vessel and a deal was brokered with Hong Kong Orient Overseas Container Line founder C. Y. Tung to lengthen the ship by several metres and add 146,152 tonnes of cargo capacity through jumboisation. Two years later the ship was relaunched as Seawise Giant.[5][11] "Seawise", a pun on "C.Y.'s", was used in the names of other ships owned by C.Y. Tung, including Seawise University.

After the refit, the ship had a capacity of 564,763 tonnes deadweight (DWT), a length overall of 458.45 m (1,504.1 ft) and a draft of 24.611 m (80.74 ft). It had 46 tanks, and 31,541 m2 (339,500 sq ft) of deck space. When Seawise Giant was fully loaded its 25 meter/81 foot draft was too deep for the ship to safely navigate the relatively shallow waters of the English Channel.[5] The rudder weighed 230 tons, and the propeller weighed 50 tons.