I am attempting to install a nest but do not have a common wire on my hvac unit, perhaps it is slightly dated. I have read that on older units the O terminal is the same as C and I can remove the green wire (fan) and use that on my O as my common wire. Since I am unable to add a net new wire, could someone kindly let me know if this is a viable solution and the best approach I should take (see pic of my setup)? Thank you.
Try the terminal between G and Y on your control board
It looks like the C terminal on your control board is between the G and Y terminals, so I would try there as your first port of call, provided your thermostat cable has spare wires in it. (It's labeled /C in the bottom row of labels, and has a wire from what likely is your air conditioner compressor running to it already.)