The most reliable serial number for Mido watches is often found on the . You will need to open the caseback (best done by a professional watchmaker) to see:
If it is a very vintage (pre-1950s) Mido, it might not have a serial. However, virtually all Mido watches from the 1960s onward have serials. No serial on a modern Ocean Star means stop—do not buy it. mido serial number check
Since no official Mido serial database exists online, bookmark these third-party resources: The most reliable serial number for Mido watches