1. Ohma may survived from his secret technique but better to *fake* die because he was hunted by unknown group (Worm)
2. In Kengan Omega Ch.1, Yamashita Kazuo reveal that he was bad at keeping secrets. So It was not good idea to tell him that Ohma is still alive.
3. After announcement Ohma's (fake) death, Worm group stopped searching him. So no one know that he still alive.