You have Nomoonaga, Akuji, Youkai, Kagemoosha, and of course the Death Oni. Akuji and Youkai are extra bosses and don't have to be defeated in order to finish the instance.
There are two bosses that you have to face: Nomoonaga, and Kagemoosha, who turns into the Death Oni when defeated. There are two optional bosses there that you can face: Akuji (4,000 health, bottom right corner of the map)) and Youkai (8,000 health, far west corner of the map). The dojo to the northwest doesn't open.
Even if you don't try Youkai or Akuji, be sure to head to the far west part of the map (take a right turn after you defeat Nomoonaga), so that you can talk to Oakheart, and do whatever quests he wants done. This version of Oakheart is easy to miss if you just do the main storyline, and you may need to finish his quests in order to advance later in the game.
There is: Akuji- Death Youkai- Life Death Oni- Storm... Just kidding, it's Death. Kagemoosha- Also Death (How many death guys are here?!? ) Nomoonaga- Death! (I mean really!!! How many!?!?!? )
Those are just the bosses, you'll also see: Kakeda Painbringer- Storm Kakeda Soulcrusher- Ice Kakeda SpiritSlave- Fire Kakeda Stalker- Balance Kogani Deathwalker- Myth... Nope, it's Death........... Wait...What??? It's Life? Dang, that's one misleading name...
There are two bosses that you have to face: Nomoonaga, and Kagemoosha, who turns into the Death Oni when defeated. There are two optional bosses there that you can face: Akuji (4,000 health, bottom right corner of the map)) and Youkai (8,000 health, far west corner of the map). The dojo to the northwest doesn't open.
Even if you don't try Youkai or Akuji, be sure to head to the far west part of the map (take a right turn after you defeat Nomoonaga), so that you can talk to Oakheart, and do whatever quests he wants done. This version of Oakheart is easy to miss if you just do the main storyline, and you may need to finish his quests in order to advance later in the game.
You are right about the Oakheart quests. All of Oakheart and Mossback quests have to be completed to be able to complete a future quest.