O2O platform may have different meanings in ecommerce context.
First, an O2O platform is a website or mobile app that acts as a middleman between physical stores and Internet users. Groupon, Restaurant.com or others coupon and reservation websites or apps may be considered as O2O platforms.
O2O or online-to-offline platforms can also refer to technology solutions that let retailers and brands take advantage of the convergence of Internet / mobile technologies and product-inventory data for increasing in-store sales with online-to-offline (O2O) marketing strategies.
For more details see O2O.