Changing a creature's base Power and Toughness until End of Turn

Question

I have a question regarding spells that change other creatures' base power and/or toughness (for example, 3/3). I'm asking because in a few days I will probably receive my Oko Planeswalker Deck from Throne of Eldraine edition, and I know there are some such cards in it.

1. When I have an instant that says target creature has base power and toughness, for instance, 3/3 and I cast it and it resolves, only base statistics change, correct? Do counters, attached enchantments or equipment, abilities, other effects stay on my creature?

Answer 1

This might get complicated, but it's covered by the layer system:

613.1. The values of an object’s characteristics are determined by starting with the actual object. For a card, that means the values of the characteristics printed on that card. For a token or a copy of a spell or card, that means the values of the characteristics defined by the effect that created it. Then all applicable continuous effects are applied in a series of layers in the following order:

...

613.1f Layer 6: Ability-adding effects, ability-removing effects, and effects that say an object can’t have an ability are applied.

613.1g Layer 7: Power- and/or toughness-changing effects are applied.

...

613.2. Within layers 1–6, apply effects from characteristic-defining abilities first (see rule 604.3), then all other effects in timestamp order (see rule 613.6). Note that dependency may alter the order in which effects are applied within a layer. (See rule 613.7.)

613.3. Within layer 7, apply effects in a series of sublayers in the order described below. Within each sublayer, apply effects in timestamp order. (See rule 613.6.) Note that dependency may alter the order in which effects are applied within a sublayer. (See rule 613.7.)

613.3a Layer 7a: Effects from characteristic-defining abilities that define power and/or toughness are applied. See rule 604.3.

613.3b Layer 7b: Effects that set power and/or toughness to a specific number or value are applied. Effects that refer to the base power and/or toughness of a creature apply in this layer.

613.3c Layer 7c: Effects that modify power and/or toughness (but don’t set power and/or toughness to a specific number or value) are applied.

613.3d Layer 7d: Power and/or toughness changes from counters are applied. See rule 122, “Counters.”

The effect you describe does not remove auras/equipments/counters from the creature. But it does affect what effect they have on the creature:

For abilities, it matters how/when the creature got them. A creature which is printed with flying (a characteristic-defining ability) will lose it due to Oko, Thief of Crowns's effect; the same holds for a creature which was already equipped with Angelic Armaments. If you equip the Armaments after Oko's ability, it will have flying.

The 3/3 effect is applied in Layer 7b (overwriting the power/toughness printed on the creature card); most effects you describe are applied in Layer 7c (but counters in Layer 7d), so they are applied later and do give bonuses (or penalties) on top of the 3/3.

Answer 2

Yes, most effects from auras, equipment and so on will stay unaffected.

In order to tell what the current characteristics of a permanent on the battlefield is, all the different effects that affect it are applied in a specific order, referred to as layers (irrelevant sections skipped):

613.1. The values of an object’s characteristics are determined by starting with the actual object. For a card, that means the values of the characteristics printed on that card. For a token or a copy of a spell or card, that means the values of the characteristics defined by the effect that created it. Then all applicable continuous effects are applied in a series of layers in the following order:

[...]

613.1g Layer 7: Power- and/or toughness-changing effects are applied.

[...]

613.3. Within layer 7, apply effects in a series of sublayers in the order described below. Within each sublayer, apply effects in timestamp order. (See rule 613.6.) Note that dependency may alter the order in which effects are applied within a sublayer. (See rule 613.7.)

[...]

613.3b Layer 7b: Effects that set power and/or toughness to a specific number or value are applied. Effects that refer to the base power and/or toughness of a creature apply in this layer.

613.3c Layer 7c: Effects that modify power and/or toughness (but don’t set power and/or toughness to a specific number or value) are applied.

So first we look at the actual printed card. It says 3/3. Then you have an effect that sets the power and toughness to 3/3, applied in layer 7b. Then, after that, you have all effects in layer 7c, where auras, equipment and other effects that grant +x/+y are applied (although many auras, equipment and other sources also grant effects that apply in other layers, like abilities, colour, change of controller or card type changes). Effects that go into layer 7b are always applied before effects that go into layer 7c, regardless of the actual order they were put into effect.

Answer 3

The answer is yes. I'll try to explain it as simple as possible:

First off, setting power/toughness never changes whether something is attached to, or stays on, your creature.

The rules tell you to do the following to calculate power/toughness (P/T) of a creature:

1. Start with the P/T printed on it, if there is a characteristic defining ability (CDA) for P/T on it, for example the creature might have a * in its P/T, then you have to read that ability to determine the P/T.
2. If there is any active effect setting base P/T, forget about what you got before and use this one instead.
3. If there is any active effect changing P/T like +1/+1 or -X/-0 where X is some number defined by the effect, apply it now.

4. Now add the change by counters (+1/+1 or -1/-1 mostly)

5. If there is any active effect exchanging P/T, do it now.(This one is added for completeness, but not mentioned in the question)

It gets slightly tricky, if two or more effects belong to a single step above. Then you apply those in "timestamp" order, that is, the older effect comes first. So if you set base P/T to 3/3, then 10/10 in the same turn "until end of turn", then the creature is 10/10, because the second effect applies second, overriding the first.