Hello Văn,
A quick question before we proceed.
Do you experience this issue when you try accessing Office Online Apps through Internet Explorer?
If the above is true, I suggest you to reset Internet Explorer's setting following steps mentioned in the following article then try accessing Office Online Apps and verify the result.
http://support.microsoft.com/kb/923737 **** **Disclaimer:**The Reset Internet Explorer Settings feature might reset security settings
or privacy settings that you added to the list of Trusted Sites. The Reset Internet Explorer Settings feature might also reset parental control settings. We recommend that you note these sites before you use the Reset Internet Explorer Settings feature.
Try the suggestion and let me know the results at your earliest convenience.
Thank you.