You don't need to look back to Proto-Indo-European for a word like this. Like many other words in Slavic, this Czech word is more likely a borrowing from Germanic prior to the merger of short *a and *o to /o/ in Common Slavonic. Indeed, I just checked and it is listed on p. 200 of Pronk-Tiethoff’s The Germanic Loanwords in Proto-Slavic, the modern standard reference for these matters.