A woman who worked for Oprah Winfrey's OWN network claims she was subjected to a hostile work environment ... because she suffers from Multiple Sclerosis.

According to a lawsuit filed Friday in Los Angeles Superior Court, Catherine Dunn was an assistant at OWN beginning in August 2008.

She claims that during her time there, she was "subjected to a hostile work environment" because of her MS. She claims she was forced to take a stress leave as well of a leave of absence because her MS symptoms were exacerbated because of the workplace stress.

When she was out on leave, Dunn claims she was replaced by "a non-disabled and younger employee."

Dunn is suing for wrongful termination, among other things. She is asking for unspecified damages.

So far ... no comment from Oprah & Co.