Disclaimer: I don't own "Crisis" or any of the Arrowverse.
Oliver searched throughout Barry's life, looking to see what could and should be changed, and after finding certain things that absolutely needed erasing and/or tweaking, went to work.
He knew that he could not save the life of Barry's mother since it was the necessary catalyst for him to become the Flash. Same as he could not stop his own father's death since it's what led him to become the Arrow. But he decided to prevent Barry's father from going to prison and for a crime he did not commit and so Barry was raised by his father.
But Oliver did not want Henry to struggle alone and therefore had Joe, along with the latter's own wife for whom Oliver rewrote things so that she never became an addict, help him in caring for Barry in the form of assisting Henry whenever it was needed, helping him cook meals for himself and his son, and babysitting Barry whenever necessary. And so Barry did not grow up with the Wests but they still remained a major part of his life.
Next, he rewrote Barry's personality so that he would not be so focused on Iris or ever selfish, and in turn rewrote Iris's own personality so that she'd never be selfish or arrogant while sometimes displaying terrible bedside manner. And so they were both better for it while more respectful towards others, including Eddie and Linda in particular.
Other changes he made involved preventing Henry's death, which erased Savitar's existence that happened due to Flashpoint, ensured that Devoe and his wife would not lose themselves in darkness and instead the former would use his immensely super intelligence to help others but the right way instead of becoming the Thinker, and prevented Orlon and his niece from being hit by the meteor and in turn, Cicada and Cicada II were never born.
But, and as much as he wanted to, Oliver knew that he could not change Ramsey Russos's fate as Bloodwork since it was necessary and needed for Barry to face his own mortality and in turn, prepare him for the Crisis.
Finally, he also wanted Felicity to be find love and be happy and, especially considering that he managed to fix her personality, found her to be a worthy match for Barry and that they were cute together. Not to mention that Barry and Iris had tried dating but decided that they were better off as friends.
But there was also Patty, who Oliver saw as having great chemistry with Barry, and so the question was about how to be fair to both her and Felicity?
A clever idea then came to him and Oliver, smirking, had Barry be with both of them in a polyamorous relationship, which started during the sixth year of Barry's own mission and post the Crisis, and seeing how happy it made all three, he was certain that he'd done well. He also made certain to have Iris find love as well.
On a side note, and while Savitar, the Thinker, and both Cicadas would never come into existence, he was unable to prevent other enemies from taking their places but did manage to make Barry and his team better prepared and equipped to deal with them while having faith that they would succeed as always.
Smiling at the work he did for Barry's reality, Oliver next decided to see if he should do any adjusting for his wife's team, the Legends.
To Be Continued
A/N In regards to Savitar, the Thinker, and Cicada II, I personally liked them as big bads but almost no one else did and I think had felt that they downgraded the series quality. And so that's why I had Oliver erase them. Same with WestAllen even though I personally ship them.
As for how the series now looks with the changes Spectre Oliver made:
S1- It remains as its written but with Barry growing up with his father instead of the Wests though they're still a big part of his life, and his love triangle with Iris never happens while they both show more respect to their S1 partners.
S2- As we remember except that Barry and Iris do date briefly but soon decide they work better as friends and so split up amicably, and Barry later gets with Patty and they never break up. Plus Henry is not murdered by Zoom while Barry never breaks up with Patty and she stays.
S3- Because Henry is saved, flashpoint never happens and Savitar in turn never exists. But a different villain does arise though the team are able to defeat him.
S4- No Thinker, though Devoe and his wife use the former's advanced genius to help the team take down a different big bad.
S5- It's written the same for the most part, but Nora's name is Dawn in the rewritten reality, she's Patty's daughter, Patty being Barry's wife, and due to future Patty and future Felicity being very open and honest with her, she knows not to trust Thawne in the future and therefore never works with him. And someone other than the Cicadas is the big bad.
S6- Exactly as it happens up to the Crisis, and afterwards, but no WestAllen while Patty is his wife. And they bring Felicity into their romance after the Crisis.
As for Iris, and due to Oliver erasing her bad behaving, annoying, and out of order moments, she's now more like her S6 self throughout the series.
Have a great day.