Word to refer to someone who takes an oath

One relevant standard legal term is deponent. From Henry Black, A Dictionary of Law (1910):

DEPONENT. In practice. One who deposes (that is, testifies or makes oath in writing) to the truth of certain facts ; one who gives under oath testimony which is reduced to writing ; one who makes oath to a written statement. The party making an affidavit is generally so called.

Another legal term very similar in meaning to deponent is affiant, which Black defines as follows:

AFFIANT. The person who makes and subscribes to an affidavit. The word is used, in this sense, interchangeably with "deponent." But the latter term should be reserved as the designation of one who makes a deposition.

Yet another potentially relevant term in Black's dictionary is jurator:

JURATOR. A juror; a compurgator ["One of several neighbors of a person accused of a crime, or charged as a defendant in a civil action, who appeared and swore that they believed him on his oath"].

Note that Black's entry for juror does not limit its meaning to "member of a jury":

JUROR. One member of a jury. Sometimes, one who takes an oath ; as in the term "non-juror," a person who refuses certain oaths.

Of these four terms, jurator is the one least encumbered with other common meanings that refer to specific legal roles, but it is a rather unfamiliar (and certainly rarely used) word.