Oyarzabal's 22nd-minute penalty – his fifth goal of the tournament – coupled with Tottenham fullback Porro's assured second-half finish secured a 2-0 win for the European champions on Tuesday (Wednesday AEST) and denied Didier Deschamps a third successive final as his 14-year-old reign drew to a heartbreaking close.
In the process Spain, who will face either England or Argentina in New Jersey on Sunday (Monday AEST), extended their unbeaten run to a record-equalling 37 games on a night when Les Bleus' big names simply did not turn up.
Bradley Barcola, preferred to Desire Doue in Deschamps' starting line-up, served warning of his intent with an early surge down the left wing, but it was the Spaniards who threatened when, after Alex Baena had drilled a free kick harmlessly into the defensive wall, Lamine Yamal latched on to the rebound to send in a deft 11th-minute cross which was ultimately smuggled away.
With star men Michael Olise, Ousmane Dembele and Kylian Mbappe largely subdued, favourites France were struggling to make an impression.
Indeed it was Spain who went ahead with 22 minutes gone when, after Lucas Digne had brought down the onrushing Yamal inside the area as he tried to clear, Oyarzabal smashed the resulting penalty emphatically past keeper Mike Maignan.
The French were dealt a second blow on the half-hour when central defender William Saliba, who had been nursing a back problem, limped off gingerly to be replaced by Maxence Lacroix, although they were briefly encouraged when Barcola cut inside and sent a 36th-minute shot over.
However, they might have fallen further behind within two minutes when Yamal played a one-two with Dani Olmo before picking out Fabian Ruiz, whose stabbed attempt was blocked by Dayot Upamecano.
Unai Simon had to race from his penalty area to intercept Adrien Rabiot's 42nd-minute through-ball for Mbappe, but the Spain keeper otherwise enjoyed an unexpectedly comfortable first half.
Luis de la Fuente's men picked up where they had left off after the break, denying France time and space and therefore the ability to belatedly establish a much-needed rhythm, to their opponents' mounting frustration.
Oyarzabal fired over on the turn and with Yamal starting to torment Digne, the 2010 winners remained on top, prompting Deschamps to replace Barcola with Doue 12 minutes into the second half in a desperate search for inspiration.
But Spain extended their lead within seconds, Porro exchanging passes with Dani Olmo before lifting his shot over the helpless Maignan.
Marc Cucurella had to get a toe to Mbappe's 67th-minute attempt to protect Simon, but substitute Ferran Torres headed wide at the other end 12 minutes from time as La Roja eased across the finishing line.
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