Octomom's oldest son claims his ex-nanny is bizarrely obsessed with him and won't leave him alone -- and he just got a restraining order to keep her away ... TMZ has learned.

According to legal docs ... Gina Bryson has been incessantly harassing 11-year-old Elijah Solomon with phone calls and text messages.

Sources close to Octo tell us, she fired Gina over the summer -- and that's when Gina allegedly began her campaign of psycho calls and texts.

The docs accuse Gina of leaking false stories about Elijah to the media -- including a story about him sexually molesting his younger sisters, and a story that he watches porn.

A judge has prohibited Gina from going within 100 yards of Elijah, or contacting him in any way.

We're told Octo is in the process of filing additional restraining order requests for two other children whom Gina's allegedly been harassing as well..