using System;

using System.Collections.Generic;

using System.Text.Json;

public class Program

{

    public static void Main()

    {

        string json = "{\"O\":{\"shortUrl\":\"www.google.be\"}}";

        var t = JsonSerializer.Deserialize<Test>(json);

        Console.WriteLine(JsonSerializer.Serialize(t));

    }

}

​

public record Test(Dictionary<string,object> O);