So, I have a quick question. I am trying to put together an anti-meta deck in modern, and am at a roadblock for this ruling. do Oath of Gideon stack each other if I have more than one on the field? Like, if a planeswalker enter's with four normally, with the four OoG make it eight when it enters?
If you can keep more than one on the field, then yes (legend rule!). Each Oath creates its own replacement effect, increasing the number of initial loyalty counters by 1 each.
The relevant ability on oath of gideon is a static ability that generates a replacement effect.
If you have multiple replacement effect trying to affect a single event the affected player chooses the order in which they apply, then they apply the first one, re-evaluate the event to see if the other effects still apply and then apply those.
In the case of multiple Oath of Gideons the application of the replacement effect does not cause the others to become invalid as a planeswalker is still ETB so if you have 4 oaths on the battlefield any planeswalker that ETB under your control will get 4 additional loyalty counters.
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However, as Rezzahan implied but didn't outright state, you can't have multiple Oaths of the same name out at one time under normal circumstances, since they're legendary. As a state-based action, you would be forced to put all but one Oath into your graveyard. You would still get the tokens from Oath of Gideon (or the other ETBs from the other Oaths), of course.
The only way to have multiple Oaths of the same name out when you land a Planeswalker is with Mirror Gallery.
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If you have multiple replacement effect trying to affect a single event the affected player chooses the order in which they apply, then they apply the first one, re-evaluate the event to see if the other effects still apply and then apply those.
In the case of multiple Oath of Gideons the application of the replacement effect does not cause the others to become invalid as a planeswalker is still ETB so if you have 4 oaths on the battlefield any planeswalker that ETB under your control will get 4 additional loyalty counters.
- H.L Mencken
I Became insane with long Intervals of horrible Sanity
All Religion, my friend is simply evolved out of fraud, fear, greed, imagination and poetry.
- Edgar Allan Poe
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The only way to have multiple Oaths of the same name out when you land a Planeswalker is with Mirror Gallery.
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