In order for Openous to replace the out of the box SharePoint form with your custom design, it is necessary that custom scripts are allowed to run in your SharePoint site. These scripts configure the appropriate web parts within your list/document library's page, so that your users enjoy the smooth experience of your custom form rendering, inline with the rest of their familiar SharePoint environment. 

Allowing or preventing custom scripts in your Office 365 sites must be done by your SharePoint administrator in the "Settings" page of the SharePoint Admin Center. In some cases, by default the scripts are enabled so you can skip this step (for example in sites -other than the organizational root one- that were created by your SharePoint admin). 

More info can be found in Microsoft's documentation here:

https://docs.microsoft.com/el-gr/sharepoint/allow-or-prevent-custom-script