last edited by Smmenen
PO may still be viable with Opal restricted, but it's greatly weakened. Not simply because Opal is a common accelerant for PO, but because of the way in which Opal functions more broadly in PO decks, and as PO decks build a critical mass.
In my experience, Opal is the card that allows Paradoxical Outcome to avoid bottlenecking on color requirements. Paradoxical Outcome can still generate gobs of mana post resolution without Opal, and in that way mimic Desire's capacity to make spells "free," but mana requirements and requisites become much tighter, and the pilot much more likely to run out of the critical color, and thus "stall," despite having plenty of colorless or off-color mana post PO resolution. And this is not simply because you get to replay your opal and tap for any color, but because you can draw more Opals, and use them like Lotus Petals for critical colors.
In contrast, in that function, Chrome Mox is very weak substitute because it requires a card to imprint, and only taps for one color of mana (in most cases), and not always your first choice.
Chrome Mox and Mox Diamond were once restricted in Vintage, but when Mox Opal was printed, I opined in my set review that I thought it was the "best Mox since Alpha," and I think that assessment remains true. I wouldn't play less than 4 Opal in any Paradoxical Outcome deck, because the marginal value of Opal is actually quite high, and it's synergy with PO is quite pronounced.
In any event, I don't understand why anyone right now would want paradoxical outcome restricted. For many months now, people have been complaining that there are no blue draw engines that can compete with Gush. Let's move forward, not backward.