Microsoft controlling open office

[Robert ...]

Hello,
 
Could you please tell me how to stop Microsoft office from tanking over my open 
office documents.
 
Every time I try to open my open office documents - Microsoft changes them to 
their format and won't allow me to make any changes unless I sign in with them.
 I don't want to sign up with them I am using your product.
 
Can you tell me how to have open office as my one and only office  program?
 
 I have windows 10.   
 
 
 
Thank  You
 
 Robert