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Is OAK Ash's secret dad?
It would make sense to keep it a secret. Think about it. Ash wants to be a Pokemon master. Having such a famous dad would just turn him into a spoiled brat and give him a huge head start. That way he would never learn the value of hard work and never become a pokemon master. Plus OAK is far too busy to live together with ash and his mom.
Oak is the richest dude in Pallet town and Delia is the most attractive woman. It would make sense that they bred at some point.