The third benefit of the War Caster feat (PHB, p. 170) says:
When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature rather than making an opportunity attack.
When casting a spell as a reaction through the War Caster feat, nothing is mentioned regarding the use of slots. So is this a free spell, or does it fall through back to the standard spellcasting rules, which would require the usual expenditure of a slot?