Oxtoby managed Bristol City from 2018 to 2021 before becoming assistant coach to Emma Hayes at Chelsea for two seasons.
The Australian has also had spells as Scotland, Birmingham City, Perth Glory and Australia Under-20s assistant.
She began her managerial career at Nottingham Forest in 2012.
Under Oxtoby, Northern Ireland lost to Norway in the Euro 2025 qualifying play-offs last year.
Last month, in what proved to be Oxtoby’s final two games in charge, they lost 5-0 to Iceland on aggregate to miss out on promotion to the top tier of the Nations League.
“It’s been an honour and a privilege to lead Northern Ireland,” Oxtoby told the Irish FA’s website upon the announcement of her departure.
“We have seen a massive transition of young players coming through and a real high performing environment created which is a huge credit to players and staff alike who have been a pleasure to work with during my time in post.
“I wish everyone the best going forward. To the fans, thank you for always supporting the journey. Northern Ireland, and the Green and White Army, will always have a special place in my heart.”
Newcastle had been without a head coach since Becky Langley left on 21 October with the club ninth in the table.
They went fully professional in 2023 and finished fifth in their first campaign in the second tier last season.
This season the top two from WSL 2 will earn automatic promotion to the top flight, while the team that finish third face a play-off with the bottom club in the WSL.
