The Bridesmaids star is marrying British TV presenter Dawn Porter and wants to celebrate with friends in the UK and friends in the US.
They couple are due to marry in September in an intimate ceremony in London with close family and friends, but they are also said to be angling for an American celebration.
An insider told Closer: "Dawn has her UK celeb mates coming to the first wedding, like Caroline Flack, but they'd love an LA bash too."
O'Dowd proposed to Porter last December, and has recently revealed that his fiancée has turned into a very controlled wedding planner in the process of organising their special day.
The pair are said to also be eager for their favourite talent show star to perform at their wedding.
The source added: "They both want Ruth Brown from The Voice to sing at the service, and Dawn's chosen an emotional song."