(1) Background: Nutritional status can influence the quality of life (QoL) of cancer patients. (2) Methods: This subanalysis evaluated the impact of an oral oligomeric enteral nutrition (OEN) protocol on the QoL of patients with oncology treatment-related diarrhea (OTRD) in a multicenter, observational, prospective study (DIAPOENO study). QoL was assessed with the Nottingham Health Profile (NHP) at baseline and after eight weeks of OEN treatment. (3) In the overall population, all the NHP categories significantly improved after eight weeks of OEN treatment: energy levels (p < 0.001), pain (p < 0.001), emotional reactions (p < 0.001), sleep (p < 0.001), social isolation (p = 0.023), and physical abilities (p = 0.001). QoL improvement was higher in patients with improved or maintained nutritional status and in those with improved consistency of stools with the OEN protocol. However, QoL did not significantly improve in patients with worse nutritional status and with worse or maintained stool consistency with the OEN protocol. QoL improved regardless of disease severity. Multivariate logistic regression analysis showed that weight change was significantly associated with improved QoL (OR 2.90–5.3), except for social isolation, in models unadjusted and adjusted to age, sex, oncology treatment, and stool consistency. (4) Conclusion: In this subanalysis, the OEN protocol was associated with improved QoL.