Once you have passed the oral portion of a checkride, do you have to retake it ever again?
Unfortunately, yes.  It is part of the checkride and if you don't finish it within the required time you will need to take the entire checkride again.

The commercial PTS shows the areas of operations that are included in the checkride, and the oral itself is given throughout the entire process.  What most people consider "the oral" is the examiner asking questions during the "preflight preparation" portion of the checkride.  This is where they ask the most questions because in all reality, this is where you as a pilot should be asking the most questions.  

There is a note in the beginning of the PTS which talks about this:


  Examiner Responsibility 
  
  The examiner conducting the practical test is responsible for 
  determining that the applicant meets the acceptable standards of 
  knowledge and skill of each Task within the appropriate practical 
  test standard. Since there is no formal division between the “oral” 
  and “skill” portions of the practical test, this becomes an ongoing 
  process throughout the test. Oral questioning, to determine the 
  applicant’s knowledge of Tasks and related safety factors, should be 
  used judiciously at all times, especially during the flight portion of
  the  practical test. Examiners shall test to the greatest extent
  practicable  the applicant’s correlative abilities rather than mere
  rote  enumeration of facts throughout the practical test. 
  
  ...




As a small aside, you don't really get two months, but rather 60 days.  This is covered in 14 CFR 61.43:


  §61.43 Practical tests: General procedures.
  
  ...
  
  (f) If a practical test is discontinued, the applicant is entitled
  credit for those areas of operation that were passed, but only if the
  applicant:
  
  (1) Passes the remainder of the practical test within the 60-day
  period after the date the practical test was discontinued;
  
  (2) Presents to the examiner for the retest the original notice of
  disapproval form or the letter of discontinuance form, as appropriate;
  
  (3) Satisfactorily accomplishes any additional training needed and
  obtains the appropriate instructor endorsements, if additional
  training is required; and
  
  (4) Presents to the examiner for the retest a properly completed and
  signed application.