Calling Excel from a C# function

I need to know how to call an external Excel function from a class. The excel formula works fine inside the main function but not in the separate class.

using System;
using System.Data;
using System.Data.SqlClient;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Excel;

namespace TestNormSInv
{
public class optionMath
{
public double testDouble(double price)
{
double result = 0.0;
//The line below produces the error !!!!!!!!!!!!!!!!!!!!!
result = WorksheetFunction.NormSDist(price);
return result;
}

public double testTriple(double price)
{
double result = 0.0;
//The line below produces the error !!!!!!!!!!!!!!!!!!!!!
result = WorksheetFunction.NormSDist(price);
return result;
}
}
class CalcExcel
{
static void Main(string[] args)
{
double result = 0.0;
Excel.Application oXL;
try {
//Start Excel and get Application object.
oXL = new Excel.Application();
oXL.Visible = false;
oXL.UserControl = false;

// This code works
result = oXL.WorksheetFunction.NormSDist(1.04);
Console.WriteLine("Normsdist 1.04: {0}", result);
optionMath OM = new optionMath();

//This does not work
result = OM.testDouble(1.04);
}
catch (SqlException e)
{
// Display error
Console.WriteLine("Error: " + e);
}
finally {
Console.WriteLine("Connection closed.");
}
}
}
}