On Jews and Oath-taking

When I served a mission in Russia, I lived for a few months in the city of Novorossiysk, along the shores of the Black Sea. Though the city itself was founded in 1840, the human history of the region stretches back far into antiquity.
Not far from Novorossiysk is the resort town of Anapa, built on the ruins of Gorgippia. This Greek city belonged to the kingdom of the Bosphorus which controlled most of the northern side of the Black Sea. Gorgippia, a wealthy city indeed, covered over 40 hecatres. Its wealth came mainly from the grain trade, but it also supplied Greece and Asia Minor with fish, fur and slaves.
Trade opportunities are what appear to have attracted the Jews to the Bosphoran Kingdom where, by Roman times, they had a substantial presence and influence.
Gorgippia's community was prosperous and seems to have had its own synagogue. Several decades ago, a rather intriguing inscription was found, which though brief, provides an unparalleled glimpse into ancient Jewish society.
I reproduce the translation given by Lee I. Levine in his book The Ancient Synagogue: The First Thousand Years.

To the Most High God, Almighty, blessed, in the reign of the king Mithridates, the friend of [?] and the friend of the fatherland, in the year 338 [= 41 C.E.], in the month Deios, Pothos, the son of Strabo, dedicated to the prayer-house, in accordance with the vow, his house-bred slave-woman, whose name is Chrysa, on condition that she should be unharmed and untroubled by any of his heirs under Zeus, Ge, Helios.

This inscription deals with manumission, or the freeing of a slave. It is a written testimony that Chrysa the slave-woman is now free and that Pothos' heirs have no claim on her.
The names of the two Jews mentioned in the text- Pothos and Strabo- indicate how Jews tended to adopt the names used by their neighbours, much like in 20th century North America, when a whole generation was named Irving and Ira.
While at first glance the typical Jewish formula of a threefold invocation of God's name might appear odd, nay, even shocking when combined with the blatantly pagan formula of an oath by Zeus, the earth and the sun, let us look at some other Jewish documents.

Maimonides, a staunch opponent of paganism and idoltary if there ever was one, in his Sefer Hamitzvot (the book of commandments) rules that swearing by astral bodies is acceptable if one has the Creator in mind.

In 1961, Yigael Yadin headed an archaeological expedition to the caves above the Dead Sea. The caves were the last refuge for some of Simeon bar Kosiva's (Bar-Kochba) rebels as they fled the Roman onslaught on Ein-Gedi. Among the astonishing finds in what became known as the "Cave of Letters" was an archive of documents belonging to Babatha, a wealthy widow and landowner in Ein-Gedi and Petra.
In the subscription to one document, we read, "I, Babtha, daughter of Simon, swear by the genius of our lord Caesar that I have in good faith registered as has been written above." Italics mine.
http://www.pbs.org/wgbh/nova/scrolls/babatha.html

In page 215 of his Hellenism in Jewish Palestine, Saul Lieberman provides a translation of a responsum by a 9th century Babylonian, the Rab Hai ben Nahshon Gaon.

Heaven forbid that one should do so (i.e. to circumvent the law) in vow or oaths, for that is a serious matter. There came to us a pious, learned old man and taught in the school: It is written (Deut. 4:19), 'And lest thou lift up thine eyes unto heaven and behold the sun' - that means to make a vow by it - 'and the moon' - that means to swear by it. If you transgress or circumvent either of them, then, 'thou hast gone astray' (ibid.) and are required to do the most severe penance. For the Lord will wreak vengeance upon you, and you will on this account be considered on a par with those who worship the sun and the moon. That is why it says further on (ibid. 26): 'I call heaven and earth to witness against you this day that ye shall utterly perish.'

Lieberman logically surmises that ignorant or crooked Jews abused a loophole in these kinds of oaths by sun and moon, which their gentile neighbours considered binding, but they themselves did not.
In fact, the closing formula in the Gorgippia inscription was standard legal fare in the Bosphoran Kingdom, and as such, seems to have lost most of its pagan connotations.

A final source from the last decade of the first century AD, though not a Jewish one, which Lieberman provided.
Martial's Epigrams, book XI.

XCIV. ON A JEW, A RIVAL POET.

As for the fact that you are exceedingly envious and everywhere carping at my writings, I pardon you, circumcised poet; you have your reasons. Nor am I at all concerned that, while carping at my verses, you steal them; for this too, circumcised poet, you have your reasons. This however, circumcised poet, annoys me, that, though you were born in the heart of Jerusalem, you attempt to seduce the object of my affections You deny that such is the case, and swear by the temples of Jupiter. I do not believe you; swear, circumcised poet, by Anchialus.

Martial seems aware of a Jewish prediliction for not taking gentile oaths seriously, and demands a stronger one, one that Jews would find binding.

For those interested in further reading on the subject, I highly recommend Saul Liberman and Lee I. Levine's books, mentioned above.