using System;

using System.Linq;

​

string[] tests = new[] { "oo", "ooo", "oooo" };

int maxLength = tests.Select(v => v.Length).Max();

foreach (string s in tests)

{

    Console.WriteLine($"\"{s}\"".PadRight(maxLength + 2) + $" -> {s.Count("oo")}");

}

​

public static class Ext

{

    public static int Count (this string s, string substr, StringComparison strComp = StringComparison.CurrentCulture)

    {

        int count = 0, index = s.IndexOf(substr, strComp);

        while (index != -1)

        {

            count++;

            index = s.IndexOf(substr, index + substr.Length, strComp);

        }

        return count;

    }

}