The Hatching O-Power is indeed the last O-Power you can get because you need all the other ones to get it. Obtaining the O-Powers are actually like a side quest that you need to complete.
Kind of a mini spoilers I guess but this is what you do to get it once you get all the O-Powers first from the strange men in Mauville City inside the Pokemon Center:
> Speak to him again after beating the Elite Four and he will invite you up to his apartment in Mauville Hills. When there, they will tell you that they are longing for someone to give power to, and learn of a powerless man across the hall. Speak to him and he will go to their apartment. Soon, the five men explain that they want to retire and they soon start to merge with the man and the man becomes Mr. Bonding. Just before he leaves for other regions to give O-Powers, he gives you the final O-Power (the Hatching O-Power). -Serebii
Hope this helped!