Question

What I doing wrong?

config.xml

...
    <helpers>
        <lacpaycs>
            <class>OS_LacPayCS_Helper</class>
        </lacpaycs>
    </helpers>
</global>
<frontend>
    <events>
        <sales_model_service_quote_submit_before>
            <observers>
                <lacpaycs>
                    <type>singleton</type>
                    <class>OS_LacPayCS_Model_Observer</class>
                    <method>lacpaycs_payment_send</method>
                </lacpaycs>
            </observers>
        </sales_model_service_quote_submit_before>
    </events>
...

then in Observer Code:

public function lacpaycs_payment_send(Varien_Object $observer)
{
    ...
    $helper = Mage::helper('laqpaycs');

and I getting error – magento tries to get helper from Mage/Laqpaycs/Helper/Data.php

How to say magento that it must get helper from OS/LaqPayCS/Helper/Data.php

Answer 1

You have a typo.

Your helper class group is <lacpaycs> but your helper factory argument is laqpaycs. Change your argument to lacpaycs and, provided that you have a class definition for OS_LacPayCS_Helper_Data at OS/LacPayCS/Helper/Data.php`, your class will be instantiated.