Oichi won't evolve till post-game according to Serebii.
I am in a similar situation with Muneshige TC, so I am going to assume it is the same for all of them unless someone can vouch otherwise. --- http://myanimelist.net/profile/dude3991 PSN/Steam: dude3991
This is a list of warlords and how to get there "evolutions". - http://www.serebii.net/conquest/bushou.shtml
Also some info quoted from a different section: Method In addition to Pokémon evolving, many of the Warlords have the ability to "evolve". This evolution causes their appearance to change and also increases the various stats of the Warlords which allow for quicker movement of the Pokémon, more Pokémon to be raised and even a different Best Link Pokémon.
To get the Warlords to evolve, you need to reach an incredibly high Link level with the Pokémon that is classified as the best Pokémon for the Warlords to be tied with. Not all of the Warlords have the ability to evolve, with it mostly being the Warlords who are gold coloured in the menus --- "Emancipate yourself from mental slavery none but ourselves can free our mind"