Squeeze your glutes and entire ab/core like your life depends on it for OHP. It helps to prevent excessive curving of the back and fixes just about all form issues.
If your form is breaking down and you started with good/perfect form, then the only reasonable explanation is that the weight is too heavy for you. Lower the weight.
If you find it difficult locking the top on a deadlift, then it’s either your back or your hands. If it was your back, then you would have injured yourself at some point, so I’d wager it’s your hand/grip. Grip strength is incredibly (I daresay most for beginners) important. Lots of people can deadlift anywhere from 50-100+ with the use of a mixed grip on a deadlift. For example, I currently do 5 reps at 205 with a double overhand grip. I can do working sets of 280 with a mixed grip and have a 1rm of 320. I have never used chalk or wrist straps, but from what I know chalk can add 20-40lb to your max, while wrist straps can add a helluva lot more to your overhand grip, seeing as how using a mixed grip gives you close to 70-80lb. Imagine not having to mind your grip at all.
In short, grip is enormously important for the deadlift. I think your grip is your weak point, and that’s why you’re having such a difficult time improving your deadlift. Sure as hell is the only reason, imo, why I can’t deadlift 300+ for working sets right now. Grip strength takes a whole lot longer to improve.
The only way to improve your grip is, obviously, working on them. Lots of ways to do that, but probably the most practical one is doing DB rows. Do perfect form DB rows where you’re not hitching anything and using entirely your back. Then do really heavy DB rows where you have to throw your body (within limit) into it. You’ll know whether the weight is too heavy or not by whether your shoulder suddenly feels anything. If your shoulder feels even mildly uncomfortable, then the weight is too much and as such your form is breaking down terribly. You’re still using your back to pull the weight up, but you’re also using your body to force out power that wouldn’t exist otherwise.
I tried doing static holds at the top and lots of lower weight deadlift to absolutely no avail for improving grip strength. My grip only improved (a 2 reps of 250 with a double overhand was the best I got) after getting to doing 95lb DB rows as described above.
Weighted chin/pull-ups and being able to do chin/pull-ups for a number of reps (14+) will also help considerably.