The actress spoke at length about her cancer battle in an interview published by Vogue Sunday ... saying she had to undergo more surgeries last month after medication she received made it so she could barely get out of bed.

The way Olivia explains it ... on the advice of her doctor, she started taking the estrogen-suppressing drug Lupron -- which she says wrecked her energy, and made it hard to do mundane tasks.

It got so bad, she says, her son Malcolm would point to her bed when someone asked him where she worked ... and, she couldn't stand her son thinking that -- so, she asked for a different treatment.

Ultimately, she says she decided to have an oophorectomy and hysterectomy -- operations that took out her uterus, fallopian tubes and ovaries, making it impossible for her to have kids.

OM thought ahead in that regard ... revealing she underwent an egg retrieval and froze several so she can still possibly have more kids with partner John Mulaney if the pair want -- noting they both feel they're not done growing their family.

As we reported ... Olivia announced her breast cancer diagnosis last month -- saying she had a double mastectomy. She said she'd had four surgeries up to that point and confirmed to Vogue this was her fifth.