What is the difference between Spare and OSS (On-Site-Spare) devices?

Functionality-wise an OSS (On-Site-Spare) and a Spare device are the same.
The difference is that an OSS (On-Site-Spare) device is sold to End Customers, a SPR ( Spare) device is sold to any   resellers  or distributors. 

OSS (On-Site-Spare) devices are also called cold spares, and are purchased by Customers.
If you are a customer, you can keep an OSS device on stock and use it to replace a defective asset of the same model type. It is recommended in an environment which cannot afford downtime. 
It is not possible to activate net new licenses on an OSS device. 
Licenses can be transferred to an OSS device from a Production device of the same model type. The expectation is that an OSS device can replace a Production device only until the RMA replacement arrives at the customer site. The licenses then have to be transferred to the RMA replacement and the OSS device will get back its part number (SKU) and can be used again as a spare. It is not supported to swap SKU between Production and OSS devices per system design. Refer to EULA . Using an OSS device as a Production permanently exhausts the concept of using the OSS "beyond the scope of the use right and/or capacity purchased" stipulated in the EULA. 

There is a procedure to convert an OSS device to Production device in case you would like to upgrade. In this case, you must pay for the conversion, because an OSS device is originally cheaper than a Production device.
Conversion from a Production to an OSS device is not supported. 
 
Spare devices are purchased only by INTL Authorized ASC Partners.
If you are an ASC Partner, you can use your Spare device to replace your customer's defective asset in case of an RMA. Licenses can be transferred from a defective production device to a replacement Spare device via Customer Support Portal, the license transfer converts the asset from Spare to Production.

Please Note: Japan operates under a specific exception process regarding the OSS swap policy.