You think there is no confusion possible? It's a very obvious knockoff of Microsoft's product. And unlike office, which is kind of generic, the use of 365 for a productivity suite is certainly specific to Microsoft.
365 is a descriptive term used widely to mean 'always available', it's too generic for a registered trademark by itself - Microsoft shouldn't have a hope in a sensible IP court.
Seems to me that open365 is different enough of office 365 that no confusion is possible anyway but I have no idea what the law says.