Spain manager Luis de la Fuente fielded a much changed squad but they dominated from the start and former Newcastle United and Leicester City forward Ayoze Perez gave the hosts the lead in the 24th minute.
"It was a dream debut," Ayoze, who scored five goals in Betis' last seven LaLiga games this season, told TVE.
"To score a goal and give an assist it's just great, I’m delighted. Every ball I controlled, every run I made, I put it all together towards my goal that is making the team.
"I tried to seize the opportunity in every practice session and in the game. Now it’s up to the coach to make the best decision for the team. It is a special opportunity. Negative thinking will only distract you from your goal."
Real Sociedad's Oyarzabal, who was one of four substitutions at halftime, extended Spain's lead in the 53rd minute with a tidy finish, scored the third on a quick counter in the 66th and got his hat-trick in the 73rd when he fired home a rebound from inside the box.
Ferran Torres wrapped a comprehensive victory eight minutes later on another counter-attack.
Spain face Northern Ireland on Saturday in their final warm-up game before they start their Euros campaign on June 15 against Croatia in Group B. They will also take on Italy and Albania.