SharePoint Office 365 : CORS issue, REST API call to other application from SharePoint Online

When trying to access 3rd party REST API (EasyVista) from SharePoint web page, it shows No 'Access-Control-Allow-Origin' header is present on the requested resource.

I've tried passing username and password in jQuery headers, I checked the access-that wasn't the issue. SO, I checked the developers tools to find that CORS is the issue.

function loadListItems() {
        var oDataUrl = "<Other applicaiton REST URL>";
        var username = "restapi";
        var password = "xxxxx"
      //  console.log(_spPageContextInfo);        $.ajax({                url: oDataUrl,                type: "GET",                dataType: "json",                crossDomain: true,                headers: {
                        "Authorization" : 'Basic' + btoa(username + ":" + password),
                        "accept": "application/json;odata=verbose"
                },                success: successFunction,                error: errorFunction
        });
}

Here is the complete error message : Access to XMLHttpRequest at '' from origin '' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource.

This is what I heard back from MS Support : In case of SharePoint Online, we need to use OAuth to get data from your custom api.

I do not know how to achieve this, any help would be greatly appreciated.

I've tried many other articles, I couldn't find a proper solution, any guidance would be much appreciated. SharePoint dev MTSharePoint