Japan's 2017 world champion won when the two met in last year's semi-finals, and she was victorious again in Odense 21-19, 21-17.
Okuhara had lost her previous seven finals - including at the 2019 Denmark Open to Tai Tzu Ying - but was too strong for the Spaniard as she secured her first tournament victory since the 2018 Hong Kong Open.
There was little to choose between the pair in a high-quality encounter with Okuhara showing great defence in the first game.
They were locked at 18-18 before Marin missed what should have been an easy put-away with her opponent scrambling.
A superb smash down the line set up three game points, and the Olympic bronze medallist converted at the second attempt before bowing to the crowd.
The second game was tight until the midway stage when the 25-year-old Nagano native established a three-point lead.
She managed to stay clear of Rio 2016 gold medallist Marin, securing victory with an emphatic smash to win on her third match point.
"I’m happy! Long time no see for a title! Compared to other opponents, Marin is faster and has a harder attack. I was focussed on defence and footwork, and my feet moved very well today," Okuhara said after the match.
World number seven Anders Antonsen survived an early scare to win the men's singles final against Rasmus Gemke.
Denmark were guaranteed a first Denmark Open men's champion for the first time in a decade with the pair, who have played one another since they were young boys growing up in Aarhus, set out their stall early with a remarkable 48-shot rally on just the second point of the match.
It was world number 11 Gemke who started the brighter, taking the first game 21-18 before running out to a five-point lead in the second.
But Antonsen wasn't to be denied as he reeled off point after point in his comeback to edge the second game 21-19.
Antonsen then showed his class in the decider as he pulled away from his friend and rival to clinch it 21-12 and become the first home winner since Jan O. Jorgensen who announced his retirement this week.
The win means Antonsen moves up to second in the world rankings with Gemke at number three.
That means the world's top three men are all Danes with number one Viktor Axelsen sitting out the competition after undergoing minor surgery.
Mark Lamsfuss and Isabel Herttrich became the first German mixed doubles players to win a BWF World Tour Super 750 or Super Series event when they defeated Chris and Gabby Adcock of England in the mixed doubles final.
Despite falling 18-21 in the first game, the Germans bounced back strongly against the married third-seeded pair.
The fourth seeds surged to big leads in the two remaining games, eventually triumphing 18-21, 21-11, 21-14 to write their name into the history books.
"I don't know what to say. It's just incredible and feels so good," Lamsfuss reacted. "We knew we could win this match, but I didn't expect to win a Super 750. It's not a normal 750, we beat some good pairs and it's just great."
Earlier, top seeds Fukushima Yuki and Hirota Sayaka won the battle of Japan to take the women's doubles title.
After cruising through the first game against Matsumoto Maya and Nagahara Wakana, they were made to work hard by the two-time world champions before eventually winning 21-10, 16-21, 21-18 in one hour and 11 minutes.
Fukushima and Hirota led 20-14 in the decider before the number two seeds saved three match points.
But the match ended on something of an anti-climax as Matsumoto served into the net.
That saw Fukushima and Hirota regain the title they won in 2018 and make a successful return to action having won March's All England Open before Covid brought badminton to a halt.
Hirota said, "We are so happy we could play this Super 750 event, it's such a big tournament and when we get back to Japan, we want to eat something very delicious, to celebrate."
The Rio 2016 bronze medallists came from a game down to win 20-22, 21-17, 21-18 and end a 45-year wait for an English triumph.
Ellis said afterwards, "This is a huge tournament, and to win a Super 750 is one of our biggest tournament wins. It’s going to be one of the best ones we’ve had."
This was their first win over the 2016 All England Open champions in five attempts.