Let me give you something better - I'll teach you how you can find it by yourself.
I'll assume that you have F&O development environment tools. Open Application Explorer and find data entities related to sales orders. You'll do it by typing a search expression to the search box on the top of Application Explorer. If you want to find data enties with names starting with "Sales" and containing "Order", you'll use type:dataEntity ^sales.*order.
Then review the list; you should spot SalesOrderHeaderV2Entity and SalesOrderLineV2Entity. That's likely what you want.
Also, look at properties of these entities and notice PublicEntityName and PublicCollectionName. These are names used in OData.
