OfferID    IncompatibleOfferID    Active(Y/N)
----------------------------------------------
1          13                     Y
1          22                     Y
2          21                     N

public function _prepareCollection()
{
    $items = Mage::getModel('offer/offer')->getCollection();

    $this->setCollection($items);
}

/** @var Varien_Data_Form $form */
    $form = new Varien_Data_Form(array(
        'id' => 'edit_form',
        'action' => $this->getUrl('*/*/save', array('id' => $this->getRequest()->getParam('id'))),
        'method' => 'post',
        'enctype' => 'multipart/form-data',
    ));

/** ETC ETC **/

$promoFieldset = $form->addFieldset('related_promo', array(
            'legend' => Mage::helper('offer')->__('Promozioni'))
        );

$relatedProdsGrid = $this
    ->getLayout()
     ->createBlock('offer/adminhtml_compatibleoffer_edit_offer_grid')
     ;

   $promoFieldset->addElement($relatedProdsGrid);