Where does the Old Testament mention "the devil"? Yesterday, New York Magazine published an interview with U.S. Supreme Court Justice Antonin Scalia, in which is he quoted as saying that he believes "the Devil" has given up taking possession of humans to make them sin, in favor of causing them to be atheists.  His reasoning is as follows:


  I mean, c’mon, that’s the explanation for why there’s not demonic possession all over the place. That always puzzled me. What happened to the Devil, you know? He used to be all over the place. He used to be all over the New Testament. 


So, to him, I guess, the fact that we don't see demonic possession on a regular basis, yet people are sinning and not believing, proves that The Devil is changing tactics - because in what Christians call the New Testament he is a regular feature.  But, as far as I know, there isn't much of an occurrence of "the Devil" in TaNa"Ch, or what Christians refer to as the "Old Testament".

I know what the standard Jewish approach is regarding "the Devil" (short version: he doesn't get a mention, and we don't believe in such a being - at least not in the way commonly talked about by Christian preachers). But according to the Christian view (across most denominations, as far as I can tell), there is some being called "The Devil" who possesses people and causes them to sin, etc. Where is he in what you call the Old Testament, and why would he just suddenly appear later?
While שטן (satan) does indeed mean "adversary," I doubt that every occurrence in the Tanakh of the word שטן should be understood as a (human) adversary as opposed to the angelic adversary whom most Christians refer to as "the devil" (from Gr. ὁ διάβολος). In fact, this isn't the standard in Jewish literature either.

For example, in the Babylonian Talmud, Seder Nezikin, Massekhet Sanhedrin 95a, Gemara, it is written,


  יומא חד נפק לשכור בזאי אתא שטן ואדמי ליה כטביא פתק ביה גירא ולא מטייה...


that is,


  One day, he (King David) went out to Shekhor Bezai. Satan came and appeared before him as a deer. He shot an arrow at him, but it did not reach him...


Now, based on the reasoning that שטן always means a human adversary, we're forced to believe that the rabbis of the Gemara wrote that a man appeared as a deer in front of David. Obviously that idea is nonsensical. Elsewhere in Jewish literature, Satan is said to assume the form of a bird (כציפרתא; San. 107a), a woman (כאיתתא; Kid. 81a), a poor man (כעניא; Kid. 81a), etc. Therefore, this particular שטן is no mere man.

Coincidentally, the New Testament refers to Satan as "the old serpent" (ὁ ὄφις ὁ ἀρχαῖος) (Rev. 12:9, 20:2), an allusion to the serpent in Gan Eden (Gen. 3). This Greek phrase has its equivalent in the Hebrew phrase הנחש הקדמוני (ha-nachash ha-kadmoni), which is mentioned in various Jewish literature, such as Tzeror ha-Mor, Sotah 9b, Sanhedrin 29a, etc. 

Jewish literature often equates Satan with the evil inclination (yezter ha-ra) and the Angel of Death (Bava Batra 16a: הוא שטן הוא יצר הרע הוא מלאך המות). But, it also equates him/ it with Sama'el, the name of an angel. For example, in Tzeror ha-Mor,1 it is written,


  נחש הקדמוני הוא שטן הוא יצר הרע הצד ציד הוא סמאל
  
  The old serpent is Satan; he is the evil inclination; he is "he who has taken venison" (cp. Gen. 27:33); he is Sama'el.



Should Satan always be understood as a human adversary? No.
Where is Satan in the OT? Many places.


As the evil inclination (יצר הרע), he influences man to sin (Bava Batra 16a; Gen. 6:5, 8:21).
As the Angel of Death, he has power to take a man's soul (תנא יורד ומתעה ועולה ומרגיז נוטל רשות ונוטל נשמה; Bava Batra 16a; Job 2:6: אך את נפשו שמור).



I see no evidence that שטן in Job 1-2 is a human adversary, especially when many Jewish sources insist it is a supernatural being.



References

1 Avraham ben Ya'akov Saba. Tzeror ha-Mor, Parashat וישלח יעקב, folio מו , second column. | Hebrew |