The Olimex USB-serial-cable - a 3.3v USB-to-serial adapter based on the PL2303XA chip - won't work out of the box on macOS.
While there are various drivers out there in the wild, my experience with these was far from optimal. Then, I finally I found my peace buying this one: https://www.mac-usb-serial.com.
Finally a reliable serial interface!
PS. I'm not affiliated in any way to mac-usb-serial.com
I bought this driver on Apr 2019, then I needed it again on July 2020. So, logging in online to download it, it popped out that apparently I need to buy it again. The licensing model is not really clear to me - but whatever, it's few bucks so I bought it again.
However, I wanted to reinstall it again on October 2020 and logging in online... surprise! I had to buy the driver again, for the THIRD time in 2 years. That doesn't seem right, but I had no choice as the support is inexistent (they didn't answer any email).
So lesson learned: the driver is great, but BACKUP THE INSTALLER BINARY and don't give for granted that next time you need it, it's there online waiting for you!