open up checkout.phtml template file * app/design/frontend/YOUR/THEME/templates/onestepcheckout/checkout.phtml right before

find the following line <?php echo $this->getChildHtml('billing_address');?>

and add this before this line

  • <?php $middlename = $this->getLayout()->createBlock('customer/widget_name')->setObject($this->getOnepage()->getQuote()->getBillingAddress())->setFieldIdFormat('billing:%s')->setFieldNameFormat('billing[%s]')?> <?php if ($middlename->showMiddlename()): ?>

    <div class="field name-middlename">
        <label for="<?php echo $middlename->getFieldId('middlename')?>"><?php echo $this->__('M.I.') ?></label>
        <div class="input-box">
            <input type="text" id="<?php echo $middlename->getFieldId('middlename')?>" name="<?php echo $middlename->getFieldName('middlename')?>" value="<?php echo $this->htmlEscape($middlename->getObject()->getMiddlename()) ?>" title="<?php echo $this->__('M.I.') ?>" class="input-text" <?php echo $middlename->getFieldParams() ?> />
        </div>
    </div>
    

    <?php endif; ?>