Costa opened the scoring for Portugal from the penalty spot
Northern Ireland fell to their first defeat in Euro 2025 qualifying as they were beaten 4-0 by group leaders Portugal in Leiria.
Carole Costa opened the scoring for the hosts with a penalty on 25 minutes, with substitute Catarina Armado finding the net either side of a Lucia Alves double.
Francisco Neto's Portugal have won their opening three qualifiers without conceding a goal.
Northern Ireland remain in second place in Group B3 despite the defeat, one point ahead of Bosnia-Herzegovina.
Watch: Portugal outclass Northern Ireland in Leiria
Sloppy start costs NI
Oxtoby, in the build-up to the game, had talked about the importance of Northern Ireland starting the game well, but they looked nervous in the opening stages and almost conceded inside the first minute.
A careless pass from Demi Vance was seized upon by Alves and she drove down the right before picking out Telma Encarnacao, whose goal bound header was blocked by Rebecca McKenna.
Jessica Silva then raced through on goal on 12 minutes, but her shot was blocked by Rebecca Holloway
A poor clearance from Jackie Burns then landed at the feet of Alves and she slipped in Kika Nazareth, who curled a shot over the crossbar.
Northern Ireland's poor start was eventually punished on 25 minutes when McKenna bundled over Alves in the box as she was rushing in on the rebound after Burns had parried Joana Marchao's free-kick into her path.
Costa stepped up and confidently tucked home the spot-kick, sending Burns the wrong way.
Portugal continued to dominate proceedings and Burns was called into action once more on the half-hour mark to thwart both Silva and Alves' low strikes, before Costa headed off target from a corner.
Alves magic helps Portugal to victory
Northern Ireland struggled to get going against a classy Portugal side
Northern Ireland enjoyed a good spell at the end of the half, but their hard work to get in at the break only a goal down was soon undone as Portugal doubled their lead three minutes after the restart.
The dangerous Alves, who had been a threat on the right-hand side all evening, produced a deft lobbed finish over Burns and into the far corner from inside the box.
Portugal were not content to rest on their laurels as they continued to push forward in search of a third goal with Andreia Norton going close as her header from Andreia Jacinto's free-kick rattled the crossbar.
Oxtoby introduced Casey Howe in an attempt to cause Portugal problems on the counter-attack and, moments after being introduced, she raced forward before attempting to find Magill, but Patricia Morais came out to smother at the Aston Villa striker's feet.
Burns once again came to rescue for Northern Ireland at the other end as she prevented Laura Rafferty from turning the ball into her own net from Alves' teasing cross.
After a lull in the game, substitute Amando scored a third with five minutes remaining, producing a similar finish to Alves by lofting the on-rushing Burns.
Alves capped her impressive display with a low finish in injury time to round off an accomplished display for the hosts and a tough night for Oxtoby's side.
Northern Ireland will have to regroup quickly from this defeat as they host Portugal in the second of the double-header at Mourneview Park on Tuesday.
