Мария

class Program
    {
        static void Main(string[] args)
        {
            string path = @"E:\file.txt";
            string text = System.IO.File.ReadAllText(path).ToLower();
            string[] array = text.Split(new char[] { '.', '?', '!', ' ', ';', ':', ',' }, StringSplitOptions.RemoveEmptyEntries);

            int[] countArray = new int[array.Length];

            char letter = 'o';

            for (int i = 0; i < array.Length; i++)
            {
                for (int j = 0; j < array[i].Length; j++)
                {
                    if (array[i][j] == letter)
                    {
                        countArray[i] += 1;
                    }
                }
            }

            for (int i = 0; i < countArray.Length - 1; i++)
            {
                for (int j = i + 1; j < array.Length; j++)
                {
                    if (countArray[j] < countArray[i])
                    {
                        var temp = countArray[i];
                        string tempSTR = array[i];


                        countArray[i] = countArray[j];
                        countArray[j] = temp;

                        array[i] = array[j];
                        array[j] = tempSTR;
                    }
                }
            }

            for (int i = 0; i < array.Length; i++)
            {
                
                //System.Console.Write(countArray[i] + "");
                System.Console.Write(array[i]+" ");
            }
            Console.ReadKey();
        }
    }