Vintage 1970s Oscar De La Renta Button Up Cottage Core Peasant Dress Size 12

I’m estimating this dress is from the 70s based on the label. No size tag but the pit to pit measurement is 18” across, sleeve length is 21” and overall length is 43” so I’m guessing it’s about a US size 12. Material feels like polyester so it almost feels like a night gown type dress but I think it’s more meant as a dress than a night gown