What’s the adjectivealform of “odaxelagnia”?

You have, odaxetic

• (adjective) Referring to a sensation of biting or itching; it is not used in the working medical parlance.

The Free Dictionary

"odaxelagniac" definitely has been used, but it looks more like a noun to me

It seems to me that, even though -ac is etymologically an adjective suffix, odaxelagniac would be more likely to be used as a noun referring to someone with odaxelagnia.

So in fact, I don't think the answer to the question in your title ("What’s is the adjective form of “odaxelagnia”?") is the same as the answer to the question in the body of your post ("What’s the word, if one exists, for someone who suffers from odaxelagnia, a form of paraphilia?"). I think a sentence like "He/she is an odaxelagniac" (with a predicate noun) would be more likely than either "He/she is odaxelagniac" or "He/she is odaxelagnistic" as a way of expressing the idea "He/she suffers from odaxelagnia".

Compare the usage of maniac: I'm pretty sure that people say things like "He's a maniac!" more frequently than they say things like "He's maniac!"

A complication: even if I am right in saying that maniac (along with -iac words in general) is rare as a predicate adjective, it does seem to occur fairly frequently in attributive position before a noun (e.g. "the maniac driver"). It's not clear that the word is actually an adjective in this context, because either nouns or adjectives can be used before nouns in English (compare "the frog prince").

Using Google search, I was able to find various examples of "odaxelagniac" being used as a noun and being used as an attributive word (I'm not sure whether a noun or an adjective) before a noun.

Noun examples:

• "I'm an odaxelagniac It means I get sexual arousal from biting"

  (Whisper.sh)

• You bit like an odaxelagniac's boyfriend!

  (in comment posted by venomlash on September 25, 2015 at 4:11 PM on the Stranger article "The Morning News: After Yesterday's Fatal Crash, Renewed Questions about Ride the Ducks and the Aurora Bridge", by Heidi Groover, Sep 25, 2015 at 9:00 am)

Attributive examples:

• "...just like that teenager I yelled at last week wasn't an odaxelagniac creep"

  ("Paranoia", by MistressMacha, published Aug 3, 2011 on fanfiction.net)

• Yeah, you're right. I'm always seeing promicuous girls around here calling themselves odaxelagniac retifists. It's a plauge.
  */sarcasm*

  (Posted June 15, 2008 by Hallucigenia in "Tila Tequila calls Herself an Asexual!?", on The Asexual Visibility & Education Network Forums)

I didn't find any examples of sentences where "odaxelagniac" was used as a predicative adjective (e.g. no examples like "I am odaxelagniac", "He/she is odaxelagniac", "They/we/you are odaxelagniac").

"odaxelagnistic" seems like it would be a "correctly" formed adjective corresponding to odaxelagnia, but I don't think it has been used yet, and I don't know if it would be a good choice to refer to an individual with odaxelagnia

If for whatever reason you want a word related to odaxelagnia that is definitely an adjective and not a noun, I think the most likely formation would be odaxelagnistic, although I can't find any evidence of it being used yet.

The Oxford English Dictionary has an entry for the adjective algolagnistic, which it says is from "algolagnia n. + -istic suffix, after German algolagnistisch (1892)." It is defined as

Of, relating to, or characterized by algolagnia; sadomasochistic.

Although I can't find a dictionary with entries for urolagnistic or coprolagnistic, Google search turns up a few examples of these adjectives.

The example sentences in OED entry for algolagnistic suggest to me that adjectives of this type tend to be used to refer to behaviors or inclinations rather than to individuals:

• 1895 C. G. Chaddock tr. A. P. F. von Schrenck-Notzing Therapeutic Suggestion ix. 175 Very often we find that algolagnistic inclinations arise at the sight of boys being whipped at school.
• 1908 E. Paul tr. I. Bloch Sexual Life of our Time xxi. 558 De Sade..collected almost all the facts..regarding algolagnistic phenomena [Ger. algolagnistischen Erscheinungen] in ethnology.
• 1914 Med. Standard Aug. 301/1 Algolagnistic manifestations are frequently the result of a lack of familiarity with the facts of science, abnormal educational or religious influences, [etc.].

"odaxelagnic" also seems like it would be a "correctly" formed adjective

As Janus Bahs Jacquet mentioned in a comment, it also seems like -ic should work as an adjective suffix in this context. Even though "algolagnic" has no no OED entry, there are a number of hits for it on Google Books, some of which correspond to adjective uses. By analogy, it should be possible to use "odaxelagnic" as an adjective.

There are a few paraphilias that end with the greek suffix -lagnia. This is actually hard to look up because the dictionaries only consistently list one as far as I can tell, that is algolagnia.

algolagnia
n.
Sexual gratification derived from inflicting or experiencing pain.
al′go·lag′nic adj.
al′go·lag′nist n.
American Heritage Dictionary

n
(Psychiatry) a perversion in which sexual pleasure is gained from the experience or infliction of pain.
ˌalgoˈlagnic adj
ˌalgoˈlagnist n
Collins English Dictionary

n.
sexual pleasure derived from enduring or inflicting pain, as in masochism or sadism.
algo•lag′nic, adj.
al`go•lag′nist, n.
Random House Kernerman Webster's College Dictionary

Collins English Dictionary gives "algolagniac" as noun.

algolagniac
n
another word for algolagnist
Collins English Dictionary

From Merriam-Webster we get the noun for the person "algolagniac"

algolagnia
: a perversion (such as sadism or masochism) characterized by pleasure and especially sexual gratification in inflicting or suffering pain.
algolagniac noun

And further down on the page we also get:

algolagnic adj

In Oxford Living Dictionaries in addition to algolagnia we get:

algolagnic
adjective & noun

(strange? probably not, see below for -ic endings being both noun and adjective)

and

algolagniac noun

A person who practises algolagnia, a sadomasochist.
Oxford Living Dictionaries

OK, now this gets very weird. My claim is that the word for a person who has one of these paraphilias is mostly the word that ends in -iac. However note that American Heritage Dictionary, Merriam-Webster and Oxford Living Dictionaries list "paraphilic" and "amnesic" as both adjective and noun, and both "paraphiliac" and "amnesiac" as both adjective and noun. How confusing is that? However searches for "amnesic who" and amnesiac who" - and "paraphilic who" and "paraphiliac who" - both show that the -iac ending word is more common for referring to people with the condition.

However a person with narcolepsy or schizophrenia is a "narcoleptic" or "schizophrenic", not "narcoleptiac" or "schizophreniac". We can be sure that a person with a condition of mania, egomania, kleptomania, pyromania has a word ending in -maniac. Paranoiac is listed in dictionaries as both a noun and adjective, with Merriam-Webster also listing "paranoic" as an alternative. I can't make heads or tails out of these rules.

My suggestion, based on what I've seen and due to the fact that these are obscure terms, would be to use:
odaxelagnia as the noun for the condition (for obvious reasons)
odaxelagniac as the noun for the person with the condition (modelled on paraphiliac and amnesiac, and also to disambiguate it from the -ic suffix which is often an adjective)

As for the adjective for the person with the condition, I'm not completely sure. All the dictionaries I've checked do not list "algolagnistic", so I'd be tempted to suggest simply what the dictionaries say, to use "odaxelagnist". However the word "algolagnistic" does come up in searches, and I feel that naturally one might be disposed to use this following the pattern of "masochist"/"masochistic" and all the other words ending in -ist. Also the addition of -ic to -ist to form suffix -istic is a very natural and intuitive use of a common English morpheme:

fascist/fascistic
nationalist/nationalistic
misogynist/misogynistic
hedonist/hedonistic
pacifist/pacifistic

Where the word ending in -ist is a noun denoting a person and the -istic is the adjective.

Based on this, even though the suffix -lagnistic isn't listed in dictionaries, you may want to use this. These are such specific words that the majority of paraphilias don't even appear in dictionaries.