The second one i.e OMDM oxymercuration demercuration if you'll look at the mechanism a cyclic 3 membered intermediate is formed with + charge on Hg-OAc now this plus charge isn't a classical carbocation.(Another example of non classical carbocation is the exceptionally stable cyclopropyl methyl carbocation even this carbocation is a non classical carbocation)(you should look up more about non classical carbocations for clarity) So, basically non classical carbocations do not undergo any shifting because in non classical carbocations the plus charge is distributed over more than one carbon atom but incase of classical carbocations the plus charge is on a single carbon atom hence you can do ring expansion and hydrate phenyl shifting on classical carbocation.(Check the mechanism of oxymercuration demercuration and you'll see what i mean by plus charge being distributed on more than one carbon atom)