Yes, you are right: fot that purpose you can apply the class 'BaseModel' from the Odoo core (the file 'models.py').

All other Odoo classes (sale.order, product.product, so any) inherit that class. For example, here the columns '_name', '_order', '_sequence' are defined.

You can define your own as well like _my_global_variable = 100. Then, you may use it in any class as self._my_global_variable. Besides, you will be able to re-define it privately for a definite model.

In many cases, however, it is better to use the model 'ir.config.parameter', and apply to it. It will let you change the value through the interface. For an example have a look at the module 'sale', the file 'res_config_settings.py'. Here you can see how to save and retrieve the global settings. In such a way, for instance, odoo base url is kept.