The idea of Olli wanting to throw cool themed parties at the nightclub worked with the character. He likes to make people happy, and he wanted to make a little bit of a name for himself. There wasn't anything wrong with either of those goals. It was just the execution of the plot which was brutal on the viewers in so many ways. Turning Olli from that sweet, caring guy who trusted Christian completely into someone stupid, oblivious, and completely unwilling to hear his lover was especially terrible of the writers. I hope someone with writing skill concentrates on rehabilitating him. The sweetness of character Jo can display so well as Olli makes up for a lot, but Olli and Christian need to be written as they were established, and were played so well.
It's fine to have the characters evolve, but it shouldn't just be a forced change to serve the plot. It should be according to their natures and how they would react to the circumstances they're in. That's real development, when the audience thinks, "Of course they'd deal with it that way, and this is a new way they'd try, and they'd listen to this advice from a friend..." It's so much better when it makes sense with the characters. Olli falling in love with Christian after hearing Christian really share his feelings, and show his well-hidden (at the time) romantic streak just seemed a natural development. Christian fighting his growing feelings for Olli for a variety of logical and illogical reasons seemed to really fit his character as well. The way they comforted and supported each other as Christian struggled to be a competitive boxer showed how strong their bonds of affection were. We weren't just told about it, we saw for ourselves.
The almost-eighteen, and then barely-eighteen Deniz Ozturk had reasons for acting out as badly as he did. Abandoned by his mother, left with a father he'd hardly seen, struggling to accept his sexuality, he behaved pretty badly. He spiraled into really badly with the cheating and the drug-taking, and it just got worse. The writers of the time made the horrible decision to make him heterosexualized, or bisexual, or whatever stupid thing they rationalized it as to themselves without explaining anything to the viewer, though Deniz never had as much chemistry with anyone as he'd had with Roman. I just console myself that through the Dark Years, Deniz never stopped thinking that Roman was hot, as much denial as he was in at various times.
The development of reformed Deniz was slow, but it built up realistically. He'd had his moments of being sweet and caring when Roman was in emotional pain, back in the mid-parts of DeRo 1.0. He was supportive of Roman's career whenever they weren't directly competing, more so than most of Roman's so-called friends. Deniz had his moments of being protective of Roman, too, when they were together.
When the real reformation of the character started, it built on those little buds of promise. Deniz started being able to have a little better perception of when other people were having trouble, and gradually began to be able to articulate his emotions. It took several months, and if he'd been just a little less completely oblivious to the fact that he was in love with Roman, too, when Roman fell back in love with him, it would have cut way down on the drama. That drama was torturous in a good way. You felt like the characters earned their then-happy ending after a long struggle. Roman had been so hurt the first time around, he had no trust. Deniz didn't mature overnight, but really worked to be able to understand and express his feelings, and understand how others felt. The character development was logical, and felt very natural. That's how it should be, taking the time to do it right, so the viewer is convinced.