So I find this quite confusing; maybe OCaml does not have octal escapes but decimal ones, and \09 is the Tab character. I haven't checked.
Instead, consider the convention of control characters, such as ^C (interrupt), ^G (bell), or ^M (carriage return). Those are characters in the C0 control set, where ^C is \0x3, ^G is \0x7, or ^M is \0xD. You're seeing a bit of cleverness that goes back to pre-Unix days: to represent the invisible C0 characters in ASCII, a terminal prepends the "^" character and prints the character AND-0x40, shifting it into a visible range.
You may want to pull up an ASCII table such as https://www.asciitable.com to follow along. Each control character (first column) is mapped to the ^character two columns over, on that table.
That's why \0 is represented with the odd choice of ^@, the escape key becomes ^[, and other hard-to-remember equivalents. These weren't choices made by Unix authors, they're artifacts of ASCII numbering.