Could Mono blue get the Oubliette effect? (An enchantment that says "When this enters, target creature phases out until this leaves the battlefield.) I know white can get it. But can blue get it as well?

No, but it can turn a creature into a different creature, like a 1/1 with no abilities.

Any chance that the 20 flip cards from Kamigawa could be errata'd to transform? I know 'flip' is a different mechanic, but it could be exactly substituted by transform (treating upside down as back-to-front), and wizards have done this kind of thing before for protoversions of future abilities (oubliette, consecrate land). It would clean up the same action word 'flip' as is used by coins and 'chaos orb' cards, and bring 20 cards into the transformverse. Plus, what gorgeous reprints they'd be!

It’s functional errata that we’ve chosen not to do.

Why was Oubliette given functional errata to use phasing, while Tawnos's Coffin, which had identical wording, was left unchanged? Is this just a matter of wanting to simplify Oubliette so it could be reprinted while Coffin will never be reprinted anyway?

That’s a good guess.

Is there a reason Tawnos's Coffin didn't also get errata'd to use phasing like Oubliette?

Yes. The card works as templated. Oubliette was changed to fit on a card so we could reprint it.

So if Oubliette is getting errata'd, does that mean Tawnos's Coffin will be too?

There are no current plans to errata Tawnos’s Coffin.

If the main challenge of reprinting Oubliette is its oracle text, wouldn't the mystery boosters have been the perfect place to reprint it without needing to deal with that?

Mystery Booster was straight pick-up and I’m skeptical we have the Arabian Nights original files.

If Oubliette is reprinted in a supplemental product, would it matter much if it uses the Arabian Nights wording even if this isn't quite the way the card currently works, rather than trying to make the wording consistent with the Oracle text? The card already exists in that form and is functional, with the intent of the wording understood by those who play and play against it. I can see a problem in a Standard set, but less so in one aimed at enfranchised players.

We don’t print cards with outdated rules text.

In the case of Oubliette or something like Chains of Mephistopheles where the wording of the card literally can't fit on a modern magic card... have you have considered creating an evergreen keyword that just means, "refer to the oracle text" or just "read the rules somewhere other than the card".

We haven’t crossed that line yet. Well, at least in black border.

Wasn't translation an issue with Oubliette? I thought the reason that it was so hard to reprint is that the text won't even fit on the card when translated to German.

It currently won’t fit on a card in English.

Would it be completely out of the question to print an Oblivion Ring with 1BB mana cost at common to appease Pauper players who want an Oubliette reprint (which won't currently fit on a paper card)? Or is printing a new break not justified by the desire to reprint an old break?

It’s not our policy to break color pie to replicate old cards with new versions.