In Shakespeare's A Midsummer Night's Dream, how does Oberon manage to "perfect" each (except for Pyramus and Thisby)?

It is true that Oberon’s actions eventually bring the play’s romantic relationships into harmony. He first attempts to unite Helena and Demetrius, for whom Helena pines, telling Puck to “anoint his eyes; / But do it when the next thing he espies / May be the lady.” Unfortunately, Puck mistakes Lysander for Demetrius and puts the love potion in Lysander’s eyes. Helena stumbles upon and wakes him, making him fall in love with her. At Oberon’s direction, Puck enchants Demetrius, but then Demetrius and Lysander fight over Helena. The abandoned Hermia is heartbroken when Lysander declares that he hates her: “What, can you do me greater harm than hate? / Hate me! wherefore? O me! what news, my love!”

Puck finally sets things right, removing the spell from Lysander’s eyes and leaving Demetrius bewitched. The lovers barely remember the night except as a dream. In fact, they are better off than they were before. Demetrius and Hermia’s father had agreed that he would marry Hermia, and Hermia had no power to resist. Now, Demetrius wishes to be with Helena, so Hermia and Lysander can stay together.

Oberon also resolves the conflict between him and his wife Titania, though he hardly plays fair. He enchants her with the love potion, distracting her from their quarrel by making her fall in love with the ridiculous Bottom, whose head Puck has turned into a donkey’s. During this time, he takes “the little changeling boy” from her for his entourage. The fairy king and queen then reconcile and bless the beds of the mortal couples.

Only in this way does Oberon improve the relationship between Theseus and Hippolyta, whose marriage seems largely unaffected by his behavior. This is interesting, considering Oberon and Titania have likely had affairs with Hippolyta and Theseus, respectively. Still, after much mayhem, the romantic couples seem to be at peace.