Hello Conor, what class do you want to extend ?
if you want to extend component it's simple, 

like this :

class ListView extends owl.Component {} // for extend to create ListView class

class MyFieldWidget extends  AbstractFieldOwl {} // for extend to create new field widget (need import this class  web.AbstractFieldOwl)

class MyWidget extends owl.Component {}  // for extend to create new tag widget such as week_days, etc.

check this out: https://codingdodo.com/tag/owl/ or https://www.odoo.com/documentation/15.0/developer/reference/frontend/owl_components.html