How do you unlock the door in the old man's house in Mossdeep City? Please don't tell me WHAT is behind it, just how to unlock it. I read a question similar to this, but that was for the original games. This is for Omega Ruby and Alpha Sapphire.
To unlock the door in the old man's house in Mossdeep City, you need to scan Junichi Masuda's secret base QR code. Once you scan it, you will be able to enter the locked room in the old man's house, which is Junichi Masuda's secret base.
Hope this helps!
Source
You have to scan in Masuda's secret base code. There's more, but that's the one I have currently.
The Secret Bases in the old man's house are special ones for the Pokemon franchise developers. They are available through the use of a QR code scan. I believe Serebii has a complete list of all those published to date.
