Recently updated my Google Pixel to Android Oreo 8.0. I can't find the "change icon shape" option. Where can I find it?
Since you did not mention the developer preview version. Assuming you are using dev preview 1.
The newly added menu that’s available on the second developer preview of Android O contains a “Change icon shape” option on the main Settings screen that’s located near the bottom of the interface, immediately above the “About” section. Tapping that option prompts a pop-up window from which you can choose the shape.