Below we have a guide for winning all contests in Pokemon Omega Ruby and Alpha Sapphire, which requires berries and cosplay Pikachu.
Begin by planting berries, preferably the super effective reduction berries, and plant and water them as needed again and again. You need 15 Rainbow Pokeblocks to max out a Pokemon, so you'll need to do this quite a bit.
The next step is to enter a contest. It doesn't matter whether you win or lose, do it to unlock cosplay Pikachu. Cosplay Pikachu is the easiest way to win ribbons by far. Give it Rainbow Pokeblocks until its maxed out all of its stats, something which wasn't really possible in the original Ruby and Sapphire, and then go the Pokemon Fan Club.
Once there, talk to the chairman who will give you all of the scarves. Give cosplay Pikachu a scarf, change its form in the green room, and that's more or less it. Simply match your Pikachu with the contest you want to enter.
Now you'll everything up to and including Hyper contests just through appeal alone. Master contests take a bit more work, and you can find out what you need to do in the video below from YouTuber Verlisify.
