The actress shared a series of pics to IG Wednesday featuring her in a hospital gown -- getting a battery of tests run on her. She also posted a video of her sobbing while lying in a hospital bed while a medical professional appeared to be consoling her.

Munn also dropped text posts explaining her cancer journey so far in greater detail -- the first time anyone has heard about this -- and admitting she's kept this a secret for a while.

Olivia was diagnosed with breast cancer in April 2023 after a doctor decided to calculate her Breast Cancer Risk Assessment score. She said she's had four surgeries since the diagnosis and has spent numerous days laid up in a hospital bed.

Munn says that in February 2023, she took a genetic test with her sister that checks you for cancer genes, including BRCA -- which she says she tested negative for. She also had a routine mammogram, which was also clear but her OBGYN suggested doing further testing due to her age and family history. Olivia credits her doctor with saving her life after the doctor found that her lifetime risk for breast cancer was 37% and a biopsy then found she had Luminal B cancer.

OM says she went in for a double mastectomy one month after her biopsy ... and she encourages other women to go to their OBGYN to have their own cancer risk assessed early and often.

Munn thanked friends and family in the post ... specifically mentioning her partner, comedian John Mulaney, for all the support he'd given her through this trying time. BTW, Munn showed up with John Sunday at the Oscars ... and nobody suspected a thing about this.

She goes on to say she kept a framed photo of John and their son Malcolm on her bedside throughout every medical procedure ... making sure it was the first thing she saw when she woke up.

Olivia didn't mention being cancer-free or in remission in her post ... so it's unclear if she's out of the woods on this front.