Once you announce that you're casting Of One Mind, no player may take actions until the spell has been paid for. Notably, opponents can't try to change whether you control a Human and a non-Human creature.

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Of One Mind). The mana value of the spell remains unchanged, no matter what the total cost to cast it was.

Of One Mind is unaffected if you no longer control a Human and non-Human creature after you've cast it.