The last time I drove a car with overdrive was 20 years ago, it was an Opel Vectra, at that time I was not very clear about its purpose, the truth is that I wasn't very interested because I have always preferred manual transmissions, I really thought it was to gain more speed and power, but your initial explanation has shown me its true purpose, and it is understandable that you rarely used it, even more if you drive mainly in the city.
The most interesting thing about your experience is that the O/D indicator is the low transmission oil level indicator, for me it was also a lesson learned, thanks for sharing.
You can imagine my surprise when I found out that the O/D flashing could be an indicator of low transmission oil. So far, I don't think there is an innovation that helps indicate low tranny oil except for the burning smell of the gear or its slippage.
I'm glad the community found my content educational. Thanks for the audience as well.