Got offered a new job with a 50% pay increase after 3 interviews, which only one of them was more technical, and it didn't involve any leet code or anything.

My friends coworker tried applying and failed
multiple times previously, so I'm feeling wayyy under qualified because my OOP knowledge sucks, I'm a self taught developer. They asked me more about engineering web solutions - how I would handle a lot of traffic , how I've designed a system where it holds a lot of requests, what do I know about databases, what engines I used and why. I'm very scared to accept, and I like my current company. What do?

Your job as a developer is always to start with the things you don't know. And move to the next one, and so one.

The sweetspot in development is learning new stuff. Cutting-edge developers are the ones who are always jumping to tasks they don't know about. Average developers keep doing what they always know with average pay and average mental activity.

If you are always learning new stuff, you will always be needed, and you will call the shots when something new needs to be implemented. Also, you'll be better just by surrounding yourself with new people and new techniques. You won't fail. I used to be a graphic designer. In two years I'm doing AI stuff. There's is nothing to difficult to learn. In an emergency, the adrenaline spike will make everything happen.

As long as you were honest, they know what they're getting.
Sometimes you just need to be brave and take a risk!
Good luck!

@Redders I was honest, but they didn't make a technical interview with me. I'm afraid that i don't know good OOP practices and design patterns. I don't use interfaces, inheritance, and so on on a daily basis. Our code base is much simplyer. How much would you think that could hinder me in the new job?
They develop primarily with symphony..

Step 1 : NEVER use session in any web project
Step 2: profit

@pionell if you can learn moderately quickly, and they have a reasonable culture, and you do a fair bit in your own time over the first month or two you should be fine, do as much as you can in advance of starting, but really OOP isn't all the hard, unless you're struggling with the fundamental concepts you should be okay.

As long as you make sure your questions are either "I'm having this issue, I've tried this and this, I read these articles but still not sure, what do you suggest?" Or "I'm building this feature, I've got two possible approaches, these are the pros and cons, which would you think will fit best with the project style?" Your team will see that you're a decent developer with some skill gaps, and not a weak developer who will need constant hand holding.

@Redders the thing is that i’m currently working as a lone developer in a company that has a lot of devs, but all of them code in different language. Whenever I’m stuck with a problem I’m left on my own woth google and SA. So I think I don’t need hand holding, but i feel like a lack a lot of fundamental knowledge. I’m getting an imposter syndrome.. :(

There are no guarantees, but it sounds like you have what it takes to adapt, and if you don't make a leap of faith at some point you will never move from the kind of work you're doing now towards more advanced stuff with either OOP, functional programming or done other methodology.