Never use google for a microsoft search!!!! LOL!!! just kidding...
1) If existing user has OEM version OS installed can they use M365 for upgrade? What is the OS? Win/7/10?
2) if they can use OEM does that mean they need to purchase a retail license ? What do you really mean by OEM???
3) I saw that M365 for business is offering Windows business edition, which edition is that? There are several flavors of Office 365 Pkgs for Business! for Enterprise! for Home! one flavor is for over 300 users and Business essentials! is for maximum 300 users.
4) Where can i find proper explanation and resource for this product? Any help would be great.
Also you should signup for a Microsoft outlook "Free EMAIL" account: https://outlook.live.com/owa/ Unless you have one already, this will help in the long run in your quest to tame the Microsoft Deluge of information. This info should get you on a straighter path!!!