Is it permissible to fast for the expiation of an oath in the days preceding the month of Ramadan (two weeks or less before the start of Ramadan)?

You can fast to expiate your oath a week, or less, or more, before Ramadan (definition). This is the most preferable and absolves one's responsibility more completely, because the expiatory fast is an obligation not subject to the prohibition of fasting after mid-Sha’ban. It is permissible to delay it until after Ramadan, but this is less preferable due to the fear of death before performing it.