<button type="button" title="<?php echo $this->__('Check out') ?>" class="button btn-cart" onclick="setcheckoutLocation('<?php echo $this->getAddToCartUrl($_product) ?>','<?php echo Mage::getUrl('onestepcheckout/index/') ?>')"><span><span><?php echo $this->__('Buy Now') ?></span></span></button>
 <script>
 function setcheckoutLocation(location,chkout)
 {
  jQuery.ajax({
                type:"GET",
                url:location,
                success:function(data){
                     window.location.href = chkout;
                }
             });
}
</script>

?>
<?php $_product = $this->getProduct(); ?>
<?php $buttonTitle = $this->__('Add to Cart'); ?>
<?php if ($_product->isSaleable()): ?>
    <div class="add-to-cart left-side">
        <?php if(!$_product->isGrouped()): ?>
            <div class="qty-wrapper">
                <label for="qty"><?php echo $this->__('Qty:') ?></label>
                <input type="text" name="qty" id="qty" maxlength="12" value="<?php echo $this->getProductDefaultQty() * 1 ?>" title="<?php echo Mage::helper('core')->quoteEscape($this->__('Qty')) ?>" class="input-text qty" />
            </div>
        <?php endif; ?>
        <button type="button" title="<?php echo $buttonTitle ?>" id="product-addtocart-button" class="button btn-cart" onclick="productAddToCartForm.submit(this)"><span><span><?php echo $buttonTitle ?></span></span></button>
    </div>
    <div class="paypal-wrapper"><?php echo $this->getChildHtml('', true, true) ?></div>
<?php endif; ?>