O/d light flashing

my 2000 ford ranger has the o/d light flashing and it shifts hard.

It means that your transmission has an internal fault, and that you are doing damage to it by continuing to drive. Within the first few pages of your owner’s manual it clearly tells you to stop driving the truck and have it repaired immediately. Failure to follow this advice will result in you having to replace your transmission.

Right now the fault is likely of the electronic variety.
If you continue to drive it, this will morph into a much more expensive mechanical repair.

If you don’t believe us, try reading the relevant text in the Owner’s Manual.
It will likely tell you the same thing.
Repairs that are deferred are invariably more expensive repairs.

You might as well continue to drive it because that flashing OD light costs around $2600 to extinguish…

Not usually, if you attend to it right away, and if you don’t go to a chain transmission shop.