Of One Mind — ruling 1

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 — ruling 2

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.

Of One Mind — ruling 3

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

Of One Mind : règles, fonctionnement et interactions (MTG)

Of One Mind is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

This spell costs less to cast if you control a Human creature and a non-Human creature. Draw two cards.

How it works : Of One Mind

Original English text — Wizards of the Coast / Scryfall

#1April 17, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#2April 17, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#3April 17, 2020· Wizards of the Coast
Of One Mind is unaffected if you no longer control a Human and non-Human creature after you've cast it.
No other card is named in this ruling.