Sharepoint list If formula

Question

I have a column that needs to return the value of another column based on if the value in a third column = S

Answer 1

You simply need to use IF statements: http://msdn.microsoft.com/en-us/library/bb862071.aspx

For example:

=IF([Column1]<=[Column2], "OK", "Not OK")

Answer 2

In addition to John's answer which seems best,

You can attach a Item Adding event to your list or you may able to use calculated columns.

Here's the code to give you an idea,

public override void ItemAdding(SPItemEventProperties properties)
{
    try
    {
        string newFieldValue = GetNewFieldValue();
        object oldFieldValue = properties.AfterProperties[COLUMN_NAME];
        if (oldFieldValue == null || oldFieldValue.ToString() != newFieldValue)
        {
            properties.AfterProperties[COLUMN_NAME] = newFieldValue;
        }
    }
    catch
    {
        //Log the error, or take another appropriate action.
    }
    finally
    {
        base.ItemAdding(properties);
    }
}