Is the extension compatible with One Step Checkout by onestepcheckout.com?

Generally yes. 

However there is one scenario where Fooman Surcharge and One Step Checkout are not compatible: When using an onsite payment method and applying a surcharge based on credit card type (ie +3% for American Express) - due to the reduced number of checkout steps with One Step Checkout, there is a risk that this surcharge will not be displayed to the customer before the payment is made. 

There are no further known incompatibilities.

In order to display surcharge amounts as tax inclusive (when using a taxable surcharge and wanting to display a tax inclusive subtotal on checkout), please use the following workaround when installing Fooman Surcharge:

1. Copy the following file to your own theme folder:

app>design>frontend>base>default>template>onestepcheckout>summary.phtml

2. Around line 93, look for this code: 

elseif($code == 'grand_total') { 
    $total_name = $this->__('Grand total'); 
}

3. Add the following code after the code above: 

elseif($code == 'fooman_surcharge') { 
    $total_name = $total->getTitle(); 
    if($checkoutHelper->settings['display_tax_included']) { 
        $fooman_tax = $total->getAddress()->getFoomanSurchargeTaxAmount(); 
        $value += $fooman_tax; 
    }
 }