Current gen models got less accurate and hallucinated at a higher rate compared to the last ones, from experience and from openai. I think it's either because they're trying to see how far they can squeeze the models, or because it's starting to eat its own slop found while crawling.

https://cdn.openai.com/pdf/2221c875-02dc-4789-800b-e7758f3722c1/o3-and-o4-mini-system-card.pdf