The printer’s manuscript has the inexplicable proping. The 1830 typesetter interpreted this word as praying. There are no other scriptural examples of “fasting and praying”; instead, the expected phrase is “fasting and prayer” (or some variant of it):
The last example (4 Nephi 1:12) is equivalent in phraseology to Omni 1:26 except for the word prayer. Similar conjuncts of fasting and prayer are found in the King James Bible:
Thus one should consider the possibility that the original text here in Omni 1:26 read “fasting and prayer”.
On the other hand, the fact that proping ends in ing argues that in the original manuscript the word probably ended in ing. It would have been very easy for Oliver Cowdery to have miswritten ay as op. Further, the Book of Mormon text does have examples that conjoin the verbs fast and pray:
And finally, “to continue in” can take either verbal complements ending in -ing or fully nominal complements:
Although the 1830 reading “continue in fasting and praying” is unique, it seems intended here. The verbal praying seems to be the closest actual word to what Oliver Cowdery wrote in 𝓟 (proping). No actual English word beginning with pro (like prophesying) even seems possible in this context, so the 1830 typesetter’s interpretation seems the most plausible emendation.
Summary: Accept in Omni 1:26 the 1830 typesetter’s interpretation of the impossible proping as the word praying.