Throughout his reign he would pursue a liberal course in politics in contrast to Charles XIV John, instituting reforms and improving ties between Sweden and Norway.

Two months later, on 5 November, he was formally adopted by the king under the name of "Charles John"; Oscar was then created a Prince of Sweden with the style of Royal Highness, and further accorded the title of Duke of Södermanland.

On 17 January 1816, Oscar was elected an honorary member of the Royal Swedish Academy of Sciences, and in 1818 was appointed chancellor of Uppsala University, where he spent one semester.

Seeking to legitimise the new Bernadotte dynasty, Charles XIV John had selected four princesses as candidates for marriage, in order of his priority:[9] Oscar would eventually marry Josephine, first by proxy at the Leuchtenberg Palace in Munich on 22 May 1823 and in person at a wedding ceremony conducted in Stockholm on 19 June 1823.

In 1839 he wrote a series of articles on popular education, and in 1841 anonymously published Om Straff och straffanstalter, a work advocating prison reforms.

If Oscar did not actively assist the Opposition on this occasion, his disapprobation of his father's despotic behaviour was notorious, though he avoided an actual rupture.

His fear lest Russia should demand a stretch of coast along the Varanger Fjord induced him to remain neutral during the Crimean War, and, subsequently, to conclude an alliance with Great Britain and the Second French Empire (25 November 1855) for preserving the territorial integrity of Sweden-Norway.

[11] In the 1850s, Oscar's health began to rapidly deteriorate; he became paralyzed in 1857 and died two years later at the Royal Palace in Stockholm on 8 July 1859, four days after his birthday.