To cast a spell as an Opportunity Attack, you have to pick up the War Caster feat:

When a hostile creature's movement provokes an opportunily attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature. (PHB 170)

There's a more detailed explanation of in the Sage Compendium:

Can you use a melee spell attack to make an opportunity attack?

You can’t if the spell attack is created by casting a spell. When a creature triggers an opportunity attack from you, you can use your reaction to make a melee attack against it. The opportunity attack doesn’t suddenly give you the ability to cast a spell, such as shocking grasp.

Each spell has a casting time. A game feature, such as an opportunity attack, doesn’t let you bypass that casting time, unless the feature says otherwise. The War Caster feat is an example of a feature that does let you bypass a 1-action casting time to cast a spell in place of making an opportunity attack. (p. 12, v. 2.1)

Having said that, if for some reason you cannot pick up War Caster (e.g., GM won't allow feats), the bolded section hints at different ways to use your spell casting ability to improve your OAs (thanks to @Darth Pseudonym for the pointer!). Namely, as long as the spell (or its effect) doesn't require an action, you can use it as part of your OA.

A few examples, available depending on your class: Shillelagh, Alter Self, Elemental Weapon, Magic Weapon.

These will, at best, improve your OA's to the level of most martial classes, but will clearly be inferior to War Caster (even a lowly cantrip will do more damage than many of these spells, as soon as you hit L5). Hence, I only mention these if War Caster is not an option.