Yeah, an old fashioned is very close to being just a diluted version of a perfect Manhattan. Or conversely, a Manhattan is a very very strong old fashioned.
Ouch, my brain hurts
Rye is king for both cocktails - however an old fashioned is not alcoholic sunny d, it has a hint of orange. Both cocktails are intended to appreciate the whiskey it is made with. Strong and complex, not sweet. The Manhattan has a lot more dilution than a traditional Old Fashioned. AKA The great old fashioned debate if you want to dive deeper into that history/argument.
Old Fashioned vs Manhattan
Sugar cube (1 cube only to 2 oz whiskey ) vs sweet vermouth to sweeten (2part whiskey to 1part Vermouth)
Orange peel vs brandied cherry
both share bitters - however historically
Fee brothers bitters vs Angostura
stir poured over a lump of ice vs stirred in a shaker glass full of ice/strained