Follow below steps to handle the OEM Order:
1, User need to create the Assembly Order, please refer to How to create Assembly Order
2, After posting the Assembly Order, use Item Reclassification Journal / Transfer Order to transfer the Finished Goods from Main Location to OEM Location.
Please refer to How to create Item Reclassification Journal / How to create Transfer Order
3, Create the Sales Order - Standard - How to create and post Sales Order