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Might be a dumb question, but can you apply to judges without going through OSCAR? Or do judges prefer you use the system and not mass-mail?

Yes, you can apply blindly.

But if a judge has an OSCAR listing, abide by its requirements.

While more and more judges use OSCAR these days, a very sizable proportion (perhaps even a majority) of them still require paper applications. And even of those judges who do use OSCAR, some of them prefer paper applications (though if this is the case, they'll usually state as much in their OSCAR post). If you're interested in applying to a judge who's not listed on OSCAR, there's usually no harm in sending a cold paper application. You also have nothing to lose (and potentially much useful information to gain) from calling anonymously to ask about the hiring schedule for X term.