Whether you control an Oko planeswalker is checked only as you begin to activate Bramblefort Fink's ability. It doesn't matter if Oko leaves the battlefield before the ability resolves or later in the turn.

Any effects that modify Bramblefort Fink's power and/or toughness without setting them to a specific number or value will apply after its base power and toughness are set, regardless of the order in which those effects were created. The same is true of counters that modify its power and toughness.