Question

How to evolve Feebas in ORAS? Gen 3 method or Gen 5?

Answer 1

You can use either method to evolve Feebas into Milotic in ORAS; trade it while holding a Prism Scale or level it up when it's Beauty condition is high enough.

Feebas also has a rather unique method to evolve into its final evolution, in fact it has two of them. First, if you attach a Prism Scale, which in Generation VI is only available in X & Y, to Feebas and trade it, it will evolve into Milotic. However, thanks to the reintroduction of Pokémon Contests, the original method to evolve Feebas into Milotic has become accessible once more.

This method has you have to use PokéBlocks to boost Feebas' Contest stats. Once you have got its Beauty to near maximum levels, when you level it up it shall evolve. It's as simple as that. To do this, you'll need to use Blue or Rainbow PokéBlocks.

Source

Answer 2

It's the Gen 3 method, although it is a heck of a lot easier in ORAS.

Especially now that Feebas is a lot easier to find, it's also a lot easier to max out it's beauty stats now that Pokeblocks no longer have 'levels', all Pokeblocks work the same. Once you max out it's beauty stat (making it look like what it is in the video), raise it up a level and it becomes a Milotic.

Hope I helped. :)
Source (video)