The 19-year-old is an academy product of League of Ireland side Cork City and joined the club in January 2024. He has gone on to feature for Southampton's U21 side on a number of occasions which drew the attention of the League Two side.
On the move, O'Brien-Whitmarsh expressed how this is a huge step up in his career as he continues his development.
"I just really wanted to get back into men’s football. I had a period back home in Ireland playing men's football.
“I played for Cork City, I spent five seasons there and one year in the first team. I played around 25-30 times so I am not naive to men’s football.
“I moved to Southampton just over 12 months ago and have been playing 21s football. I have had the odd time training with the first team.
"I also made my debut in the Carabao Cup. I feel I can offer something positive here and this was the perfect opportunity for me."