Sound Ó

 

• Proto-Slavic, ó was a long close-mid back vowel /oː/.
  
• Think of it as a slightly tenser, longer version of o in “note” (without the English diphthong).
• 
  
• Over time:
  
• Polish: /oː/ shifted to /u/ but kept the spelling ó.
• Czech: /oː/ merged with /u/ in many cases, spelled ů.
• Russian & other East Slavic: length distinction disappeared; ó became /o/.
• South Slavic: often stayed as /o/, with length lost or preserved depending on dialect.
• 
  

English         | Proto-Slavic | Polish   | Russian   | Slovenian

----------------|--------------|----------|-----------|----------

mountain        | *góra        | góra     | гора      | gora

horn            | *rogъ        | róg      | рог       | rog

leap/jump       | *skókъ       | skok     | скок      | skok

mole            | *mólь        | mól      | моль      | mol

house           | *dómъ        | dom      | дом       | dom

light           | *světьlъ     | światło  | свет      | svet

three (dim.)    | *trójь       | trójka   | тройка    | trojka

salt            | *sólь        | sól      | соль      | sol

year            | *rokъ        | rok      | год       | leto

king            | *królъ       | król     | король    | kralj

god             | *bogъ        | bóg      | бог       | bog

frost           | *mrózъ       | mróz     | мороз     | mraz

fortified town  | *gordъ       | gród     | город     | grad

field (half)    | *pólъ        | pół      | пол       | pol

trace           | *ślědъ       | ślad     | след      | sled

circle          | *krúgъ       | krąg     | круг      | krog

tar/pitch       | *smólь       | smoła    | смола     | smola

So when reconstructing Proto-Slavic, ó was not /u/ — it was a pure, long “o” sound, like holding the o in “more” but without adding an “r” at the end