So, I can't find an answer to this question:
Can you take an Oracle as my Squire, with the feat from Inner Sea Knights? The only requirement seems to be that the class is required to have a class feature that grants proficiency in all martial weapons.
The Battle Mystery has such a feature, and therefore so does the class.
Am I required to SELECT the feature? Or is the fact that it is part of the class sufficient?
A squire can only take levels in a class that grants proficiency with all martial weapons as a class feature
Oracle class gives you this proficiency only if you take the Skill at Arms revelation. So the squire can not choose the oracle class, unless he's taking this revelation at the same time.