From the Wikipedia page [1]
The Crimean Oblast (Ukrainian: Кримська область, Kryms'ka oblast' ; Russian: Крымская область, Krymskaya oblast' ; Crimean Tatar: Qırım vilâyeti) was an oblast (province) of the former Russian SFSR (1945–1954) and Ukrainian SSR (1954–1991) within the Soviet Union. Its capital was the city of Simferopol.
The Crimean Oblast replaced the Crimean ASSR on June 30, 1945.
The oblast was transferred from the Russian SFSR to the Ukrainian SSR on February 19, 1954.
Sevastopol was a closed city due to its importance as the port of the Soviet Black Sea Fleet and was attached to the Crimean Oblast only in 1978.
Following a referendum held on January 20, 1991, the Crimean Oblast was given back its pre-World War II status of Crimean Autonomous Soviet Socialist Republic on February 12, 1991, by the Supreme Soviet of the Ukrainian SSR.[1]
In 1992, the Autonomous Soviet Socialist Republic was transformed into the Autonomous Republic of Crimea within Ukraine, with the exception of the city of Sevastopol whose status, once again due to its strategic importance as the port of the Russian Black Sea Fleet, remained disputed between Ukraine and Russia until 1997, when it was agreed that it should be treated as a "city with special status" within Ukraine.