///свойство ViewModel
 public NotifyTaskCompletion<List<item>>  itemsList{ get; set; }

//в конструкторе заполняем свойство


 public PersonsViewModel()
        {      
             itemsList = new NotifyTaskCompletion<List<item>>(initialitemlist());


 private async Task<List<item>> initialitemlist()
        {
            List<item> ListItemtemp_temp = new List<item>();

            string connectionString = "###";

            using (OracleConnection connection = new OracleConnection())
            {
                try
                {
                    connection.ConnectionString = connectionString;

                    using (OracleCommand command = connection.CreateCommand())
                    {        
                        string sql = $"sqlquery";

                        command.CommandText = sql;

                        if (connection.State != ConnectionState.Open)
                            await connection.OpenAsync().ConfigureAwait(false);

                        DbDataReader reader = await command.ExecuteReaderAsync().ConfigureAwait(false);
                        if (reader.HasRows)
                        {
                            while (await reader.ReadAsync().ConfigureAwait(false))
                            {
                                ListItemtemp.Add(new item
                                {
                                    property1 = Convert.ToString(reader["column1"]),
                                    property2 = Convert.ToDateTime(reader["column1"])
                                });
                            }
                        }
                    }
                }
                catch (Exception ex)
                {
                    //MessageBox.Show(ex.Message);
                }
                finally
                {
                    if (connection.State == ConnectionState.Open)
                        connection.Close();
                }

                return ListItemtemp;
            }
        }
        }

 ItemsSource="{Binding itemsList.Result ,IsAsync=True}"