Oberon is King of the Fairies, master of Puck, and husband of Titania (in a seemingly open relationship).
There are a couple of ways to read Oberon's character. At times, he can be a compassionate and benevolent softie. Why do we think so? Because he feels so sorry for Helena that he uses his magic to help her land Demetrius, and he also goes out of his way to make sure that each of the young Athenian lovers is paired up with a suitable partner. He even blesses the happy couples' marriage beds so they won't have ugly kids. Aww.
On the other hand, Oberon only helps the lovers out after he's had a good laugh at their expense. At times, he also acts like a jealous, power-hungry jerk who's willing to trick and humiliate his own wife in order to get his way. It doesn't look so good when Titania refuses to hand over her foster child, so he sprinkles love juice in her eyes and makes her fall in love with an "ass" and be distracted enough to give up the little "changeling" boy. Even though Oberon eventually takes pity on Titania, he only reverses the spell after he gets his way.
Either way you read Oberon, one thing is clear: The Fairy King really likes a good joke, which is why he's chosen mischievous Puck to be his servant. Also, Oberon's not above abusing his powers to get a few laughs.
Oberon is also the fairy world's biggest player (except for maybe his wife, Titania). Although he's partnered with his Fairy Queen, he's known for having had torrid affairs with other women. We know this because Titania accuses him of sleeping with a string of beauties, including Hippolyta, who's described as being Oberon's former "warrior love" (2.1.73). We also learn that Oberon had a thing with a country girl named Phillida and even went so far as to disguise himself as a shepherd so he could hook up with her (2.1).
Oberon seems to pursue romance as if it were a favorite sport or hobby, which tells us that he (like Theseus) has a thing for making conquests out of women. Oberon's promiscuity also shows us that you don't need magic "love juice" to quickly fall in and out of romantic relationships.
As we know, Oberon is completely obsessed with Titania's foster child and demands that she hand him over ASAP (2.1). Like we said, when Titania refuses, Oberon breaks out all the stops until he gets his way. What's the deal?, you ask. Well, Oberon never comes out and tells us what motivates his desire for the little boy, but we can look closely at the text for some possible answers. According to Puck, Oberon is jealous because Titania spends all her time lavishing the kid with her attention and ignores Oberon:
And jealous Oberon would have the child
Knight of his train, to trace the forests wild.
But she perforce withholds the loved boy,
Crowns him with flowers and makes him all her joy (2.1.24-28)
There's also a suggestion here that Oberon is on a major power trip. Puck tells us that he wants the boy to be his servant, which may be a way for him to demonstrate his power over Titania. At one point, Oberon calls Titania a "rash wanton" and asks "Am not I thy lord?" (2.1.65). Translation: "I'm a man and your husband so you should do whatever I say." At other times, Oberon whines like a little boy who isn't getting his way, even asking, "Why should Titania cross her Oberon? / I do but beg a little changeling boy / to be my henchmen" (2.1.122-124).
Whatever motivates Oberon's jealousy, one thing is certain—he's ruthless when it comes to getting his way.
We also know that Oberon and Titania have been clashing a lot and that their big "brawls" have been very destructive. Titania tells us that the fights have been so violent that they've disrupted the seasons and the weather, which has caused devastating winds, rain, and flooding (2.1). As a result, crops have been ruined and there's been a shortage of food for humans. As Titania admits, "this same progeny of evils comes / from our debate, from our dissension" (2.1.119-120).
Why does this matter? Well, King Oberon and Queen Titania's negative impact on the natural world gestures at the realities of power in the 16th century. In Shakespeare's day, rulers may not have been able to control or impact the weather, but their actions, policies, and behavior had the potential to make the lives of ordinary people miserable.