Update : the latest test result with chrome show that chrome autofill functionality HAS CHANGED. My current chrome version is 65.0.3325.181 on mac, not sure in what version the change was made. Chrome autofill in this version triggers field onchange events and therefore triggers ajax refresh.
One Step Checkout does not support browser autocomplete because of the following reasons: browser autocomplete fills fields unpredictably, it does not fire field onchange events and its functionality may differ from browser to browser.
You can turn autocomplete off with the following, <form autocomplete="false">in app/design/frontend/base/default/template/onestepcheckout/checkout.phtml
PS. Chrome used in previous versions the tag autocompletefill = "off", however that does not work in current versions, now you need to use autocomplete = "false" As this seems to be in constant change, every recent couple of months something changes, please feel free to look for updated information on google in this case this KB article seems to not contain latest information. I will update when I get reports of issues/solutions.