OSS is an online SAP service portal which will have all
kinds of updates about SAP patches and latest note
information.
Its a knowledge base of SAP for all the SAP users. It contains all the required information needed for all the activities in SAP and also the fixes for error you get in SAP.
OSS notes mentioned in the upgrade guide is nothing but the information guides to be followed when you have any errors or any sort of information related to upgrade.
Please follow the following link to access the notes.
http://service.sap.com/notes.
Your SAP team will have a login id called as S-user id which starts with S000.... and enter the relevent password to login to service.sap.com/notes
Please go through all the SAP notes mentioned in guide for getting better understanding of upgrade.
Please get back in case if you need more information.
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