The pressure was immense on La Roja to produce not only a win, but a performance after a dismal goalless draw against Cape Verde in their Group H opener on Monday.
It was one of the biggest shocks in the tournament's history - but Spain hit back at any critics with a classic showing of quick passing, attacking football and lots of goals.
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Yamal was back in the starting XI after an electric cameo last week and immediately showed why he is one of the best players in the world. He poked home Oyarzabal's low, fizzing cross on a tight angle for his first World Cup goal on his first World Cup start.
Oyarzabal's quickfire double soon put Spain out of sight - all before the first hydration break in the 25th minute. His first was a scrappy poke home at the back post, before a more intentional finish two minutes later.
He should have completed his hat-trick after collecting a poor back pass from Mohammed Al Owais, but his first-time hit pinged off the top of the crossbar.
Oyarzabal and Yamal were replaced at half-time, which a smart move from Luis de la Fuente - who celebrated his 65th birthday on Sunday - with more trying tests to come.
While Spain were less intense after the break, they remained dominant, but it was an unfortunate own goal that added the fourth. Marc Cucurella's initial effort from a flicked-on corner was initially well-saved by Al Owais, but the rebound hit Hassan Al Tambakti and into the back of the net.
La Roja thought they had a fifth in added time as Ferran Torres turned home Fabian Ruiz's cross. However, after a lengthy VAR review, the goal was ruled out for offside.
It sends Spain top of Group H ahead of Uruguay's game against Cape Verde at 11pm on Sunday, while Saudi Arabia go bottom.
The key moments from Atlanta...
1: Lamine Yamal is involved within seconds as Abdulelah Al Amri nods away his right-wing cross.
11: GOAL! Yamal scores his first World Cup goal with a poked finish on a tight angle.
21: GOAL! Spain score a scrappy second as Oyarzabal pokes home.
23: GOAL! Two minutes later, Oyarzabal scores again as he turns the ball past the goalkeeper from close range.
36: Oyarzabal pings an effort off the top of the crossbar in pursuit of his hat-trick after a Al Owais error.
49: GOAL! Spain extend their lead to four through Hassan Al Tambakti's own goal.
90+2: GOAL! Torres turns home Spain's fifth, but there is a VAR review for offside.
90+5: NO GOAL! After a lengthy VAR review, Torres' goal is ruled out.
