Is it authorized to rate somebody the same rank as you for an OER?

Are you speaking of being Intermediate Rater?
AR 623-3

Intermediate Rater:
2–6. Rules for designating an intermediate rater (DA Form 67–10 only) This paragraph does not apply to DA Form 2166–8, DA Form 1059, or DA Form 1059–1. An intermediate rater will only be included for specialty branches (Chaplain Corps, JAGC, and AMEDD), when there is a level of technical supervision between the rater and senior rater, in dual supervisory situations, and when the rater’s immediate supervisor would be the logical senior rater, but does not meet senior rater eligibility requirements as outlined in table 2-1. Under unique circumstances, requests for an exception to policy may be granted. Written requests for an exception to policy, endorsed by the first commanding General Officer (or equivalent) in the organization, will be submitted to HQDA in
accordance with the guidance contained in paragraph 2–6c. a. An intermediate rater will be an officer of the U.S. Armed Forces, U.S. Coast Guard, or allied armed forces, or an employee of a U.S. Government agency (including nonappropriated fund employees). In addition, the intermediate rater
will—
(1) Be senior to the rated officer in grade or date of rank. A civilian intermediate rater has no minimum grade requirement but will be officially designated on the established rating scheme.

Senior Rater:
Tbl 2-1
To Senior Rate 1LTP/CPT Senior Rater must be O–5 LTC/Lt Col/MAJP
I believe these address your concern.

I for one, would not have wanted to be rated by a peer of my same rank nor would I have wanted to rate either. TIG is not predominantly looked at when this rated officer's OER is reviewed for command, a position or promotion. He/she deserved someone of higher rank and that rater should be in his/her direct chain. May I also add that when I was in a position under a civilian I truly appreciated that I was rated by a senior military officer with input from the civilian supervisor.

Your local Inspector General will always have the proper guidance available: "AR 20-1 Inspector General Activities and Procedures." The IG IS the proponent for all Army Regulation. You can make an appointment, call in...even send an anonymous letter. Bottom line absent more detail = No. Without a formal written Rating Scheme, you can not rate anyone not directly under your supervision and/or without personal knowledge of that individual's duty performance. Some wiggle room may exist due to certain circumstances, but I would contact the IG to make sure. Otherwise, you can probably look this up online via AR 20-1 and navigate to the appropriate subject material. The issue as given by you would have me advise the Commander is incorrect to request this. During my own 30yr career, I was rated by a person equal in rank who held the title of "Section Chief" and I worked for him (he had higher time in grade). He was, of course, intimately familiar with my contributions and was listed on a proper organization Rating Scheme. JCH, Colonel, Inspector General (Ret).

I have done this as the preparer of the rating. Usually, when I have a better understanding of the individual than the person directly over the senior person due to my proximity to the individual. Once completed, and forwarded for approval, modifications were made as necessary and the rating then become the approving officer's, no longer mine. Of course the write-up was very confidential. Maybe that is just the Navy way, but it worked for me and the individual. Lots of strange "stuff" came my way as Special Projects Office manager at the command.

It is never appropriate to be the rater of someone of the rank. An obvious conflict of interest, even if there was none intended. Doesn't matter what branch or service. It should ALWAYS be a senior. I put up with that crap on a Surface Warfare Officer board where one of the board members was of equal rank and not yet qualified. I should have complained, but it would have caused MORE heartburn than the situation itself. Afterwards, I told same person that they needed to finish ALL parts of the old Warfare section of the SWO Qual package before I would entertain them at a SWO board. He (and others) didn't and had to wait until I left. Equal justice under the law.(Sorry about that.) Same thing goes for FITREPS/OERS. If you have to, you go to the rater's superior. In some places, like USAF used to do, that was a good thing.(I think they stopped it-a who knows who endeavor.) Good. A bulls##t practice.

Interesting issue. I've been out for awhile, but as I recall that would be a no go. A-equal grade B-how can you objectively rate someone you have little working knowledge of? Re: AR guidance-I have no idea. See your S-1. Good luck.

I've been out of the Army too long to advise you on manuals and such. For me the key phrase is: "per CDR''s guidance."

Yes. I had another captain rate me on deployment. I think they need to have a higher date of rank

USCG Persman Chapter 10

Chapter 2, AR 623-3 will provide the backup regulation as stated by BG below.