Hartsock, who competed for Sean Lowe's heart and then starred on The Bachelorette's ninth season, threw a baby shower for Oteri this past weekend, Us Weeklyreported.
"Renee's shower was [Sunday] and it was so much fun having everyone there for her! I had it at Trophy Cupcake & Party Room in Seattle and it was a great space to decorate however I wanted," Hartsock told Us.
"The theme for Renee's shower was beachy since she is from Florida and loves the beach -- not to mention, I do too! -- so that made it easy to decorate."
Oteri, 33, is pregnant with her second child -- her first baby boy with new husband Bracy Maynard, whom she wed in March. Oteri is already mom to a nine-year-old son Ben from her previous marriage.
Hartsock, 28, and Oteri both currently reside in Seattle, WA. Hartsock lives with her fiance Chris Siegfried, whom she got engaged to at the end of her The Bachelorette edition. Oteri moved for Maynard's job.
Guests at the baby shower reportedly included former The Bachelor bachelorettes Clare Crawley and Lauren Solomon -- who had vied for Juan Pablo Galavis' attention along with Oteri during his edition that aired earlier this year.
"A few of the girls from Juan's season were there to support her like Clare and Lauren," Hartsock said. "Cassandra Ferguson and Alli Restko were invited but couldn't attend. Molly Mesnick made it, of course."
Ferguson and Restko also appeared on Galavis' The Bachelor season, while Mesnick competed on the thirteenth season of the franchise.
"We all just had a blast mingling, drinking mimosas, onesie decorating and playing a game with safety pins on necklaces," Hartsock told Us. "Renee is the cutest pregnant lady ever and it's so fun to see her so happy!"
"A HUGE thx to my beautiful friend + amazing host @DesHartsock for throwing the sweetest baby shower ever!" Oteri tweeted Sunday. "Love you much!"
Guests at the baby shower reportedly used the hashtag #BeachBabyJax numerous times, potentially hinting at the baby's name.