Here Oliver Cowdery initially wrote “according as their brethren”, a clausal fragment. Later, with somewhat heavier ink flow, Oliver supralinearly inserted the auxiliary verb had. This correction may have occurred when Oliver read the text back to Joseph Smith. There is clearly a need for some verbal element here. Another possibility would have been to insert did rather than had.
Elsewhere there are 20 examples of “according as” in the text, and each instance is followed by a subject and a predicate. Seven of these examples take the past-tense perfect auxiliary had:
There are no examples of “according as” where the auxiliary verb is do, so odds are the inserted had here in Alma 25:14 is correct. Note by the way that the ellipsis after the auxiliary verb seems appropriate; it would seem odd, although not impossible, for the text to repeat the language of the immediately preceding text:
The corrected reading in 𝓞 probably represents the original text. Some kind of auxiliary verb is needed here, while ellipsis of the rest of the predicate is expected. Of the possible auxiliary verbs, had seems to be the most plausible.
Summary: Accept in Alma 25:14 Oliver Cowdery’s corrected reading in 𝓞: “and they did also bury their weapons of war according as their brethren had”.