Hi, if you are using JS or you are returning an action to open the form you can add `flags` for this, unfortunately there is no `flags` field in the python model of `ir.action.act_window`.

JS:

 var action = {
                type: 'ir.actions.act_window',
                name: 'Action Name',
                target: 'new', //use 'current' for not opening in a dialog
                res_model: 'target.model',
                res_id: target_id,
                view_id: 'view_xml_id',//optional
                view_type: 'form',
                views: [[false,'form']],
                context: {//your context
                         },
                flags:{
                mode:'readonly'
                } // default is 'edit'
            };
        this.do_action(action);
 

in python code (assuming that your button has `type='object'`):

@api.multi

def your_button_name(self):

return {

                'type': 'ir.actions.act_window',
                'name': 'Action Name',
                'target': 'new', #use 'current' for not opening in a dialog
                'res_model': 'target.model',
                'res_id': target_id,
                'view_id': 'view_xml_id',#optional
                'view_type': 'form',
                'views': [[false,'form']],
                'context': {#your context
                         },
                'flags':{mode:'readonly'} # default is 'edit'
            };