<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="sales_order_place_before">
       <observer name="company_module_observer_check" instance="CompanyModuleObserverCheck" />
   </event>
</config>

class Check implements MagentoFrameworkEventObserverInterface
{

    /** @var  LoggerInterface */
    protected $logger;
    /**
     *
     * @param LoggerInterface $logger
     * @param array $data
     */
    public function __construct(LoggerInterface $logger) {
        $this->logger = $logger;
    }
    public function execute(MagentoFrameworkEventObserver $observer)
    {

       //TOTO i want do things like below
        $condition // condition will be got from an API service
        if($condition){
             return $this;
        }else{
            return // Go back to cart
        }
    }
}