Near the end of this passage, Oliver Cowdery initially wrote crime in the original manuscript; then somewhat later he inserted the plural s, but the level of ink flow was not as heavy. The adding of the s could represent a later correction resulting from Oliver rereading the text to Joseph Smith after this part of the text has been initially dictated. Another possibility is that later Oliver consciously made the change to crimes since earlier in the passage there is an occurrence of the plural: “for there was a law that men should be judged according to their crimes”.
Elsewhere in the text, the earliest sources give instances of both singular and plural for crime, with seven in the singular and ten in the plural. In one of these cases, an original singular crime was incorrectly replaced with the plural crimes (see under Alma 1:10). In another case, all the extant sources read crime, but the original may have been crimes (see under Alma 50:39 for discussion of this case of possible emendation). As far as Alma 30:11 is concerned, we note that when the associated verb is punish, we can have either singular crime or plural crimes:
In the first passage, both instances of the plural crimes seem more appropriate than a singular crime would. But in the second passage, the text could read in either the singular or the plural: “him have I punished according to the crime(s) which he hath committed”. In Mosiah 29:15, the critical text will follow the invariant textual reading for all the textual sources (including 𝓟, the earliest extant source), thus accepting the singular crime. Similarly, here in Alma 30:11 either singular crime or plural crimes will work, so we accept the corrected reading in 𝓞 as the reading of the original text. Nonetheless, the possibility remains that the crime that Oliver Cowdery initially wrote in Alma 30:11 could have been the original reading and that the correction to crimes was a conscious attempt to make both occurrences of crime(s) in the passage read in the plural.
Summary: Accept in Alma 30:11 the correction in 𝓞 from the singular crime to the plural crimes in “therefore a man was punished only for the crimes which he had done”; the possibility remains that the original text read in the singular and that Oliver Cowdery changed the singular to the plural since earlier in the passage the text reads in the plural (“for there was a law that men should be judged according to their crimes”).