<a_onestepcheckout>
    <reference name="choose-shipping-method">
        <action method="setTemplate">
            <template>a/l/onestepcheckout/shipping_method.phtml</template>
        </action>
    </reference>
</a_onestepcheckout>

<b_onestepcheckout>
    <reference name="choose-shipping-method">
        <action method="setTemplate">
            <template>b/l/onestepcheckout/shipping_method.phtml</template>
        </action>
    </reference>
</b_onestepcheckout>

// either a_onestepcheckout or b_onestepcheckout depending on condition
<onestepcheckout_index_index>
    <update handle="a_onestepcheckout" /> or <update handle="b_onestepcheckout" />
</onestepcheckout_index_index>

public function addSomeHandle($observer) {
    $action = $observer->getEvent()->getAction();
    $actionName = $action->getFullActionName();
    if ($actionName == 'onestepcheckout_index_index') { //if on your desired page
         $layout = $observer->getEvent()->getLayout();
         if (CONDITION GOES HERE) {
             $layout->getUpdate()->addHandle('a_onestepcheckout');
         }
         else {
             $layout->getUpdate()->addHandle('b_onestepcheckout');
         }
    }
}