Code :-

Step 1: Pass the custom flag in context.

<field name="o2m_field" context="{'is_from_o2m_field': True}">
<tree>...</tree>
</field>

Step 2: Inherit the relational model of our o2m_field and extend the unlink method like below.

def unlink(self):
if context.get("is_from_o2m_field"):
# write your custom code to call the desired function.
pass
return super().unlink()