I understand this issue arose over the past 2-3 years, but I haven't seen any solution.
A workbook created on a Windows PC using Office 365 - transferred to Mac (Yosemite installed) - opens in read-only mode in Office 365 for Mac.
The workbook doesn't have a "/" in its name. I made sure there is read/write access for the file and also the folder. It contains macros, some of which work, others will have to be redone.
After saving it with a different name, I may work as usual with it like updating, saving and closing it.
After reopening it, it's again in read-only mode.
After all this time there must be a solution for the problem.