We can't tell, without a lot more code than that - and if the source is out of focus you could have done
anything! And just saying "gives me an error" doesn't help at all without knowing what error, and where in the code! Remember, we can't compile that in isolation to even begin working out what the problem might be - and we have no idea what you expected it to do either!
I strongly suggest that if the video is out of focus and the explanation is inadequate, then you scrap it and start looking for something more professional, or ask the author for explanation - most YouTube development videos are poor quality because the author has no idea how to make a video, no idea how to teach, and more often than not no idea how to code either. Monetization is the key to YouTube riches, not ability or quality.
If you have to stick with this video (and it's probably a bad idea) then break up the code:
var x = Controls.OfType<oscillator>();
var y in x ...
and look at what type x and y are actually going to be in VS - that should give you a starting point to work out what is going on.