I think Ogata has been working for the Feds the entire time. He was assigned to infiltrate Lt. Tsurumi's faction to determine the location of the gold and prevent the fractioning of the country over it. When his position became untenable he switched sides. He's been providing information about all of the factions and major players.
I do think Ogata really likes Asirpa and sees a lot of parallels between the two of them. Both orphans with questionable fathers and close connections to their grandparents; I think he is trying to give her a choice away from all this crazy that he wishes someone had given his younger self.
That being said if I had to badly explain this series to someone it would be "Indigenous girl and her merry band of psychos go on a treasure hunt" and IMO Ogata is both best boi and the biggest psycho. Of the murders of his family members his brother's is the most chilling as it seems like is was done out of pure spite. With his mother is seemed like it was a mercy to both her and the rest of her family while his father's seemed to be based on vengeance. I think he was already working with the Feds at the time and also used it to gain Tsurumi's trust.
I just wonder about the identity of his chain of command. I would be so pleased if Noda brought in the other well known and still living shinsegumi member, who had worked for the Meji government and had assignments in Hokkaido. It would bring things full circle with Hijikata very nicely as well, which seems like Noda's style after we found out about Tsurumi's undercover work.
Anyways that was a lot! I've been following this series for a long time and this chapter was the one that caused me to delurk. Can't wait to see how things go down in the next one!