The Battle Oracle gains
Choose one weapon group that embodies your mystery. You are trained in all martial weapons of that group.
On your character sheet this looks like "Trained in martial swords" or (more correctly but less realistically) "Trained in shortsword, rapier, longsword, greatsword, [...]".
Trained in martial weapons is a specific benefit that encompasses all martial weapons, not just some. If you look at classes that provide proficiency with all martial weapons, they similarly exclude the implied word. Rogues, by contrast, begin Trained in rapier, sap, shortbow, and shortsword but similarly would not qualify.
The Weapon Proficiency Feat would provide Trained to all martial weapons (because the Oracle already has all simple) and qualify you for the Marshal Archetype (assuming you have Diplomacy or Intimidation).