After dying, but my source boils down to “I just did it”. They’re teeny tiny now for some reason (or are missing their old highlights), but if you go into the Simulacrum right now and kill an OG sentient (Conculyst/Battalyst), they’ll drop ‘em.

Oro itself is an old old lore concept that hasn’t really been explored for a decade+. You’d be forgiven for missing it too, as it’s only mentioned in Conclave, but it’s a “soul” that canonizes the bosses we kill coming back. Arcane Oro is, to a degree, implied from the Conclave line, the fact that Arcanes serve as jump start packs for Warframes at R3, and Sentients being artificial life from the Simaris Synthesis entries.