The reading “made oath” here in Mosiah 19:26 seems odd without the indefinite article an, especially given the occurrence of “made an oath” in the previous verse. Elsewhere in the Book of Mormon text, whenever we have the expression “to make ... oath”, oath is either preceded by an (two more times) or the (eight times). The two other examples of “make an oath” refer to the oath that Zoram makes to Nephi and his brothers:
Note further that in Mosiah 19:26, since there is no prior mention of the oath, we expect the indefinite article an in modern English.
Elsewhere in the scriptures, the only time we get oath without some kind of determiner (such as an, the, my, his, thine, their, this, and so forth) is when oath occurs in the plural. Yet even in the plural there is only one occurrence of oaths that takes no determiner:
But the reading “made oath” in Mosiah 19:26 is probably not a mistake for made oaths since Limhi makes only a single oath.
There is also evidence in the manuscripts that the scribes sometimes omitted the indefinite article an:
Thus an could have been dropped when “made an oath” was copied from 𝓞 into 𝓟. Of course, 𝓞 is not extant for any part of the book of Mosiah.
On the other hand, the term “to make oath” (that is, without the indefinite article an) has been common in legal language since Middle English, with the following sampling from the online Oxford English Dictionary:
Thus the reading “made oath” in Mosiah 19:26 is quite possible. Consequently, the critical text will accept the unique reading “made oath” here in Mosiah 19:26, even though it might be a mistake for “made an oath”.
Summary: Accept the unique instance of “to make oath” in Mosiah 19:26: “and also Limhi … made oath unto the king of the Lamanites”; although the indefinite article an may have been lost from this phrase during the early transmission of the text, the critical text will accept the reading without the an since there is considerable evidence for this phraseology in English, from Middle English to the present.