I can see it too, I am on the Plus plan and don't think I have any special developer privileges. Selecting that option for me changes the URL to https://chatgpt.com/?model=o1-preview

I tried a fake Monty Hall problem, where the presenter opens a door before the participant picks and is then offered to switch doors, so the probability remains 50% for each door. Previous models have consistently gotten this wrong, because of how many times they've seen the Monty Hall written where switching doors improves their chance of winning the prize. The chain-of-thought reasoning figured out this modification and after analyzing the conditional probabilities confidently stated: "Answer: It doesn't matter; switching or staying yields the same chance—the participant need not switch doors." Good job.