oos-check.php

<?php

// The ID of the current product
/** @var int $id_product */

// The selected combination ID
/** @var int $id_attribute */

// The current selected quantity
/** @var int $quantity */

$oos = \Product::getQuantity(
        $id_product,
        $id_attribute,
        null,
        Context::getContext()->cart,
        null
    ) < $quantity;