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I think Oleana used to be a Team Skull member before she left them and "cleaned up her act" to find work in Galar and eventually become the VP of Macro Cosmos. I like to think Rose took pity on her like he did Bede at one point and took her under his wing, hence why she's so devoted to him; she basically owes him her life. The fact that all of her Pokémon are native to Alola (in the regional dex) points to this as well.
