[5] Taking advantage of the civil war in Germany between the Hohenstaufen claimant Philip of Swabia and the Welf candidate Otto IV, Ottokar declared himself King of Bohemia in 1198,[6] being crowned in Mainz.

In 1199, Ottokar divorced his wife Adelheid of Meissen,[7] a member of the Wettin dynasty, to marry Constance of Hungary, the young daughter of Hungarian King Béla III.

[4] Ottokar was quickly forced back into Philip's camp by the imperial declaration of a new duke of Bohemia, Děpolt III.

Ottokar's reign was also notable for the start of German immigration into Bohemia and the growth of towns in what had until that point been forest lands.

Ottokar was married first in 1178 to Adelheid of Meissen (after 1160 - 2 February 1211),[12] who gave birth to the following children: In 1199, he married secondly to Constance of Hungary (1181 – 6 December 1240),[12] who gave birth to the following children: The Milanese mystic Guglielma (1210s – 24 October 1281) claimed to be a Princess of Bohemia[15] and has therefore been identified as a daughter of Ottokar and Constance with the name Vilemína or Božena, but there is an absence of any corroborating Bohemian documents.