Here both the 1830 edition and the printer’s manuscript read “and they do observe to keep his commandments”, so the original manuscript must have read as such. Spacing between extant fragments indicates that to keep was probably inserted supralinearly in 𝓞. The original text of the Book of Mormon has 11 examples of “observe to keep the commandment(s)”, but none of “observe the commandment(s)”. In Mosiah 4:30, the 1837 edition accidentally omitted the infinitive phrase to keep, which appears to be what happened initially in 𝓞 for this passage in Helaman 15:5. For a complete discussion of “observe to keep the commandment(s)”, plus a list of its examples, see under Mosiah 4:30.
It should also be pointed out that there are a few cases where the direct object for the verb observe is the semantic equivalent of commandments, but not that word; in fact, in all these instances, the passage actually has some form of “keep the commandments” (marked below in each case with an arrow):
Summary: Following both 𝓟 and the 1830 edition, we can conclude that in Helaman 15:5 𝓞 undoubtedly read “they do observe to keep his commandments”, even though to keep was probably inserted supralinearly in 𝓞.