Another source of "official" spelling would be whoever is licensed to distribute and translate the work into the English-speaking regions. In this case Crunchyroll uses "Oga" and "Ryo" for both the licensed manga and anime. The downside of using licensed translation spellings is if a few years later a different company gets the license, retranslates things and pick a different spelling, then that becomes the new official spelling.
I've not heard about the licensed translated manga by Crunchyroll.
Yes, that's why I am also taken a little bit aback before whether I should use the macrons for their names or not. I would have understood if their names would be spelled as Ohga and Ryo, as opposed to Ōga and Ryō. However, I would argue though that both of their names use hiragana and not katakana writing.
Before I made any changes, I consulted some references to check whether my plan would be appropriate or not
Some references though tell us that some words drop the macron outside the context of language books.
If you ask me, I'll not strongly defend the spelling of Ryō given that it is the proper romanization of his name and that (/ou/) is clearly not (/o/). They are entirely different. On the other hand, with regards to Ōga's, I'm not pretty sure about that since the reference I gave above also gives us an idea that (/oo/ = /oh/ = /ō/; /ou/ = /ō/).
I would say in the absence of an "official" source, using the above references to come to a conclusion would have made perfect sense.
But we do have official sources. If for example the official source were to say Kaisaki's English name is "Arthur" (not actually, just an example), then his English name would be Arthur. And if there is an official source providing the English spelling for おおが, then we should follow the official source.
In case where there are different official sources with different spelling, we should define a clear process of which official source has higher precedence. In this case, Crunchyroll's licensed translation differs from Comico's marketing material (meant for the Japanese market), so I would say the question should be whether we go with an "originalist" approach and follow Comico's marketing material, or do we go with the "localized" approach and follow whoever is licensed to translate and distribute the English edition of ReLIFE (in this case it would be Crunchyroll).
We aren't going to spell Tokyo as Tōkyō either, even thou (/ou/) is clearly not (/o/).