I definitely don’t think you have to stick with an O. I know a family with three kids who had a girl with an S name, then a boy with an S name, and gave their third and last child an A name. No one thinks twice about it or even notices.
If you really like the “O” sound but want another initial: (I think this might be a great way to go because it holds the “o” theme without forcing you to use a name that seems very unusual/uncommon next to your other kids’ names.
[name_f]Olivia[/name_f], [name_u]Owen[/name_u], & [name_f]Chloe[/name_f] <3
Olivia, Owen, & Sophie (or Sophia)
Olivia, Owen, & Zoe
Olivia, Owen, & Nola
Olivia, Owen, & Fiona
Olivia, Owen, & Noah
Olivia, Owen, & Jonah
Olivia, Owen, & Leo
Olivia, Owen, & Theo
Olivia, Owen, & Logan
Olivia, Owen, & Emily
Olivia, Owen, & Amelia
Olivia, Owen, & Ava
Olivia, Owen, & Stella (or Ella)
Olivia, Owen, & Scarlett
Olivia, Owen, & Emma
Olivia, Owen, & Nora
Olivia, Owen, & Opal
Olivia, Owen, & Oakley
Olivia, Owen, & Eli
Olivia, Owen, & Jack
Olivia, Owen, & Asher
Olivia, Owen, & Miles
Olivia, Owen, & Jude
Olivia, Owen, & Levi
Olivia, Owen, & Oscar
Olivia, Owen, & Orion