On the scans, the names (Rantarou and Ouma) are written with ロウ/オウ respectively—is there a reason why they're read as ō, or is it just plainly ou?
They’re both long “o” sounds, just being elongated from different initial characters: ロ (ro) and オ (o) in this case. Like how イ is used to elongate the long “a” sound.
ウ is used to elongate the long o (or short u!) sound across the board pretty much (Ruruka’s family name is pronounced アンドウ for example), and is slightly different than the double オ you’d have in Oowada or Oogami (and also technically in Osaka, though it’s almost never indicated in English or if it is it’s denoted the same way Ouma’s オウ is). If you want an example of this not in the front there’s Toyanoo (トヤノオ) from DRK 2 and possibly Yokoo (ヨコオ) from the student council. It’s really tough to hear the difference if you aren’t used to it though.
Elongated sounds are important in Japanese because this language is at max capacity for homophones as it is. ショウケイ is “short rest/recess” but ショケイ is “execution”. Makes a difference which you ask for in a trial. (Also there are multiple other words that use these exact same pronunciations, so when I bitch about out of context sound bytes this is why.)
ANYWAY, I write it as “ou” because Hepburn romanization 4lyfe and to distingish between the “oo” sound.