how long did you have the Oto? They tend to be not-so-hearty fish unfortunately. He'll want a buddy or 2 - if you have a quarantine tank for the new otos, you can acclimate your current oto to it and let them enjoy the company while they're in quarantine.
For us, we feel otos tend to die when alone.. we had 2 tanks that had 2 otos each (all we could get from the LFS) and the day after 1 died, the other did - in both tanks.. no sign of disease & perfect water temp and chemistry.
I think this may have been unrelated to the number of otos, Dave. I only have 2 in a tank, and they're quite happy.