Swift Swim will become Marvel Scale.
Oblivious will become Competitive.
When a Pokemon has 2 possible abilities, its abilities are allocated to slots. The first slot's ability, let's call it Ability A1, will always become Ability A2 when evolving. The second slot's ability, Ability B1, will always become Ability B2 when evolving. This is Milotic's case; Oblivious will always become Competitive.
There are other cases, like a Pokemon has only one ability and evolves into something that has 2 possible abilities, then it'll be 50/50.
Bulbapedia - Abilities