oppa.

Headword: 
oppa.
Principal English Translation: 

twice (see Karttunen)

Orthographic Variants: 
ocpa
IPAspelling: 
oːppɑ
Alonso de Molina: 

oppa. dos vezes.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 77v. col. 2. Thanks to Joe Campbell for providing the transcription.

Frances Karttunen: 

ŌPPA two times / dos veces (M) In two attestations C does not mark the initial vowel long, but T consistently has the reflex of Ō, and Z marks it long once out of two times. In T ŌPPA has been reanalyzed as a unitary lexical item and a second -PA attached to yield ŌPPAPA. In Z ŌPPA also seems to have been lexicalized and from it has been derived ŌPPATICA 'for a second time.' X has a different derivation, OCPA< OC 'another' and -PA, but with the same sense. See ŌME, -PA.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 180.

Lockhart’s Nahuatl as Written: 

particle. ōme, -pa.
James Lockhart, Nahuatl as Written: Lessons in Older Written Nahuatl, with Copious Examples and Texts (Stanford: Stanford University Press and UCLA Latin American Studies, 2001), 228.

Attestations from sources in English: 

oppa (adverb) = a second time, twice
Daniel Garrison Brinton, Ancient Nahuatl Poetry: Containing the Nahuatl Text of XXVII Ancient Mexican Poems (1877), 160.

Oppa icuitl quiqua. Itechpa mitoa: in aquin tla itla oquitemacac, azo itla qualoni, anozo tilmatli: ye no ceppa quitlani = He eats his excrement over again. This is said if someone gives something to another, such as food or a cape. Then he asks for it back. (central Mexico, sixteenth century)
Thelma D. Sullivan, "Nahuatl Proverbs, Conundrums, and Metaphors, Collected by Sahagún," Estudios de Cultura Náhuatl 4 (1963), 102–103.

ocpa = twice (less standard than oppa)
yn oppa = twice
Robert Haskett and Stephanie Wood's notes from Nahuatl sessions with James Lockhart and subsequent research.

itlan xaquj in metlatl, in atl, in venchioaliztli: auh ximotetlacamachiti, maca oppa tinotzalo: iehoatl in pillotl, in velnenotzaliztli, in nezcaliliztli, in tlaimacaxiliztli, in mauhcanemjliztli: auh njman ie iehoatl in iocuxcanemjliztli = Be diligent with the grinding stone, the chocolate, the making of offerings. And be obedient; do not be summoned twice. Nobility is the good doctrine, the way of prudence, the way of reverence, the way of fear, and then the way of peace (central Mexico, sixteenth century)
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 6 -- Rhetoric and Moral Philosophy, No. 14, Part 7, eds. and transl. Arthur J. O. Anderson and Charles E. Dibble (Santa Fe and Salt Lake City: School of American Research and the University of Utah, 1961), 217.