.NET Fiddle

CSharp · Console

/Program.cs

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;
	}
}