Two separate verses in Parashat Behar address the prohibition of ona'a. The first (Vayikra 25:14), which refers to cases of selling or buying, tells us that neither the buyer nor the seller may indulge in ona'a. Although the ArtScroll translation reads, "Do not aggrieve one another," it seems that "ona'a" here translates more precisely as "fraud," rather than aggravation. The Gemara in the fourth chapter of Bava Metzia details the exact parameters of fraud, and includes in this prohibition dishonest practices such as gouging the buyer and overcharging.
Three verses later (25:17), the Torah again warns against committing ona'a against one's fellow and adds in this context that one should "fear God." The Gemara (Bava Metzia 58b) explains this verse as referring to ona'at devarim (verbal ona'a) and gives various examples of such improper behavior. For example, one may not remind a ba'al teshuva or a convert of his previous lifestyle. Additionally, if someone wishes to buy a certain product, one may not refer him to a store knowing that it does not stock such merchandise. Rav Yehuda added that a person may not pretend he is a customer if he has no intention of buying.
Although the same word ("tonu") is employed in both verses, it seems that the second instance, which deals with ona'at devarim, does refer to aggravation, rather than fraud. This second verse appears to forbid causing one grief, rather than fraudulent practices. Thus, the Rambam defines this prohibition as follows: "We are enjoined... not to say things which hurt and anger another person" (Sefer Ha-Mitzvot, Lo-Ta'aseh 251).
Accordingly, the Rambam (ibid. 250-251) enumerated two separate mitzvot – monetary ona'a (fraud), and verbal ona'a (aggravation). The Rambam juxtaposed these two prohibitions in his list of mitzvot, whereas the Yere'im separated them completely. One section of the Yerei'im is entitled, "Monetary prohibitions, etc.," and another section is entitled, "Prohibitions involving someone who does evil to God and mankind, without stealing or deriving monetary benefit." The prohibition of fraud is included in the former category (127), while the prohibition against causing aggravation is included in the latter group (180).
Although some Rishonim combine both these prohibitions together into a single mitzva, this very likely stems from their different method of counting mitzvot, rather than from any similarity in definition between them. Rav Perla (Sefer Ha-Mitzvot Rabbenu Sa'adya Gaon, vol. 2 p. 111a) pointed out that two separate mitzvot of ona'a were enumerated in connection with fraud and aggrievance of a convert.
The Mishna states in Bava Metzia (ibid.): "Just as there is ona'a in business, there is verbal ona'a." Inasmuch as these are two separate, independent laws, what does the Mishna seek to convey through this equation ("Just as… ")? The Bach (Tur, C.M. 228a) explained that one might think that ona'at devarim need not be treated as severely as cases of monetary fraud, and the Mishna therefore emphasized the similarity between these two prohibitions. In fact, the Gemara gives a number of reasons why ona'at devarim constitutes a more severe prohibition than fraud. Monetary gain from fraud can always be returned, whereas grievances caused by words can never be undone. Fraud involves money, but ona'at devarim affects the physical being. Also, the phrase, "You should fear God" is written in connection with aggravation, and not with respect to fraud.
The Chasdei David (Bava Metzia, Tosefta 3:13) pointed out that the severity of ona'at devarim may have led one to apply the punishment of makkot (lashes) to such cases. Fraud, like any other monetary crime, can be repaid, and thus the perpetrator does not receive makkot. But since one cannot "repay" a victim for his aggravation, we might think that ona'at devarim entails a punishment of makkot. The Mishna therefore compared the two prohibitions, thereby indicating that makkot applies in neither case, since only prohibitions involving physical action are punishable by makkot, whereas ona'at devarim is violated verbally.
Surprisingly, the Maharam of Rottenburg (responsum 785) ruled that one is indeed liable to makkot for this prohibition, since it cannot be repaid. The Beit Yosef (C.M. 1) suggested that perhaps the Maharam meant only that the Chakhamim instituted makkot for this prohibition, while Biblical law would not call for a punishment of makkot, as no action is involved.
The Gemara (Bava Metzia 59a) cites Rav Chanina ben Rav Idi as commenting that since the Torah forbids aggrieving "your fellow," it follows that this prohibition applies only to people who observe Torah and Mitzvot. The Bach (ibid.) cites the Mordekhai's position that since the Torah employs the word "fellow" only in connection with ona'at devarim, but not regarding monetary fraud, we might conclude that the Gemara limits only ona'at devarim to observant Jews, whereas the prohibition against monetary fraud applies to all Jews. However, if this were true, fraud would turn out to be more stringent than ona'at devarim. But haven't we already seen that causing aggravation is at least as strict – if not stricter – a prohibition than fraud?
Obviously, this problem leads us to one of two conclusions. We may either disregard Rav Chanina's statement and include all Jews in both prohibitions, or accept his position and apply it to fraud, as well, and thus neither defrauding nor aggravating an unobservant Jew would violate a Torah prohibition.
Both the Rambam and Rav Yosef Karo omit the opinion of Rav Chanina, and apparently adopted the second approach. The Rama (Shulchan Aruch, C.M. 228:1) ruled, "There are those who say that the prohibition against causing aggravation applies only [when doing so] to God-fearing people." The Arukh Ha-Shulchan modifies this position and comments that one may commit ona'at devarim against a sinner as this may cause him to perform teshuva. If a person's intent is totally honorable, he may aggravate someone in order to cause him to improve his ways. If so, then perhaps we could defraud him, as well, if our intention is to bring him to greater observance of Torah and mitzvot. The Bach (op cit.) quotes an opinion that since it is obvious that the laws of fraud are less stringent than the laws of ona'at devarim, one may defraud a sinner, as well. However, the Bach himself is quick to point out that he poses this thesis only theoretically, and he is reluctant to allow following this position as a practical matter.
The Ritva advances a particularly novel interpretation of Rav Chanina's statement, claiming that it refers specifically to causing aggravation to one's wife. While this approach may not seem compatible with the text of our Gemara, and the editor of the Mossad HaRav Kook edition in fact suggested that the Ritva had a different text, the gemara does continue with details about the importance of not causing aggravation to one's wife. Of course, everyone agrees that this law applies to others, as well, but we must exercise particular care with regard to our spouses.