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Something I found really beneficial to keep my lower back straight when doing OHP was to tense my butt as hard as I could. This stops your lower back from arching.
Also make sure you are pushing the bar up in as much of a straight line as you can.
But yes, as someone else said, drop the weight until your form is better.
Edit: I see someone posted videos, which probably include what I said.
Early morning this morning, rowing to warm me up followed by some difficult overhead press.
Any tips for making that easier? It honestly feels like my back isn't straight at all when I'm doing it...