Production sources tell TMZ ... the 47-year-old is officially returning to 'RHOC' as a friend of the show -- more than a decade after walking away from the series in 2013 as a fan fave.

We're told Bellino -- who starred in seasons 5 through 8 as a main cast member and appeared as a guest in season 14 -- started filming Tuesday evening with the other housewives as production on Season 18 just began.

The biggest news going into this season down in the O.C. ... producers are obviously ready to jump all over the drama between Bellino and Beador -- who now have one thing in common ... dating the same dude in John Janssen.

Here's the deal ... Alexis recently started dating John who Shannon was in a relationship with from July 2019 until November 2022 -- when she says he broke up with her one week after filming on Season 17 wrapped.

SB said she felt blindsided by the breakup ... and AB dating her former flame probably isn't making her feel much better. They're taking getaways together and Alexis seems to have a good relationship with John's kids ... something Beador has publicly said she did not.

And, that's just the tip of the iceberg ... because even before Bellino and Janssen got together, Beador had been quite clear about her dislike for her new castmate.

TMZ broke the story ... just before Shannon's DUI arrest in September 2023, she was overheard trash-talking Alexis at a restaurant -- which Bellino says was over a defamation lawsuit Beador's ex-husband filed against her and Tamra Judge. Bellino also told us she's not involved in that lawsuit.

Bottom line ... it stands to reason there's a lot of messy and complicated feelings between these two, and Bravo knows exactly where to put them -- on a hit TV show, of course!