This is a really fun question !
I'm kinda torn. I like the idea of Ohana and Ko as a couple. He is age appropriate for her and they have a history where he has been her most loyal, supportive and caring friend.
On the other hand, I'm really liking the Ohana and Tohru dynamic, especially the way in which the writers at PA Works are setting up their relationship. Tohru doesn't just like Ohana because she is a cute girl. He took special notice of her because of her actions which showed a determined and independent nature. She is also outspoken and doesn't hold back, just like him. I think he respects her spirit , her generous and positive nature and her willingness to go the distance to accomplish what she believes in. I think they have similar temperaments. That said at this point in Ohana's life, he is a just a bit too old for her.
Too bad, they would make a great couple. There relationship would have a bit of the Mr. Darcy and Elizabeth vibe of Pride and Prejudice, the proud young man who find himself irresistibly drawn to the intelligent, independent and spirited young woman.