Effects that modify a creature's power and/or toughness, such as the effect of Festive Funeral, will apply to the creature no matter when they started to take effect. The same is true for any counters that change its power and/or toughness.

Oko's Hospitality overwrites all previous effects that set the creatures' base power and toughness to specific values. Any power- or toughness-setting effects that start to apply after Oko's Hospitality resolves will overwrite this effect.

You may cast Oko's Hospitality even if you control no creatures. You'll just search for Oko, the Trickster.