When I see the tag:
Love Triangle
I'm usually out, I never liked them in any story.
Now let me go against all that I just said, to say: Best. Love. Triangle. EVER! It turned out to be an all out Love War.
I realized what made it so good was the time jump. So the story talks about the Past and Present, what happened when the APP launched and then it jumps to 4 years later. I feel like the Leads (because there is no Main/Second) switched roles, which makes it so good.
In the past it was Sun Oh in the driver seat with Hye Young taking a passive role, loving from a distance. Although I feel Sun Oh started off unethical, with Jojo having a boyfriend and knowing his BFF liked her. However, you really saw him fall for her and truly love her.
Can I pause here to talk about my love/hate relationship with Jojo, just as Sun Oh's pursuit in the beginning wasn't a righteous one, she didn't really stop him either (cheaters always make me cringe). Her story is definitely sad and you see her fight through, not giving up. I truly felt her fear when it came to loving someone else, but the immaturity of feelings at that age *sigh*. I feel like if she had her mom, someone she could talk with, her choices would be different, but we wouldn't have a KDrama then would we? lol.
Back to the Love War, everything changed when we jump 4 years into the future. Sun Oh with someone new, horrible, but then here comes Hye Young taking a chance, okay I see you! Hye Young isn't going to stand on the sidelines this time. He goes old school, pursuing her without the APP, not even caring if she rings his alarm. Then you have Sun Oh who is passive on the sidelines now (I get it she broke his heart) but honestly still loves her after everything, hating himself for that.
And then it ENDS!
Now I'm left here without closure. Taking a realistic look, if she did go back to Sun Oh, how would the relationship look? for 4 years he was left believing she didn't like him, their relationship would need constant reassurance for Sun Oh (hence the reliance on the APP to confirm ones feelings). That's not easy to live out, its almost too broken to fix, all because of Jojo's fear. Buuuut the Chemistry, sooooo good!
Hye Young is a more realistic, healthy option, but I'm going to need more to be sold. I mean it's delicate now because you want both guys to be happy, if we can see Sun Oh accept the truth and Jojo can sort out her true feelings and be open giving them both closure, then up the Chemistry with Hye Young, I could buy it.
Whichever way it goes, I think this is a triangle I can love.

Spoiler Hypothesis!
Can I also add how I think one of the side stories in this Drama is going to play out?
Duk Go. The creator of Love Alarm. I don't see him as being dead. I know the guy on the stage wasn't him, but I don't think he died. I think he's switched teams, I think he sees the bad that came from the APP and is now a protester, specifically the guy driving the van at the end. I'd love to see him be strong, since he was so shy, and take back the company changing it for the better somehow.