Inadequate breastfeeding can lead to significant morbidity, mortality, and long-term adverse health outcomes. Key messages promoting optimal breastfeeding are integral to Essential Nutritional Action, aimed at improving children's nutritional status, especially those under the age of two. However, there is a dearth of data regarding the extent to which lactating mothers adhere to these key messages and the associated factors. Therefore, this study aimed to assess the level of practice of key messages for optimal breastfeeding and identify associated factors among lactating mothers living in the Boset District, Oromia, Ethiopia. A community-based cross-sectional study was conducted among 418 lactating mothers from January 1st to February 28th, 2022. The participants were selected using a systematic random sampling technique, and data were collected through interviewer-administered questionnaires. The collected data were entered into Epi Info version 7 and exported to SPSS version 25 for analysis. Binary logistic regressions were performed to examine the association between independent and dependent variables. Adjusted Odds Ratios with a 95% confidence interval were used to estimate the strength of the associations and statistical significance was declared at a p-value < 0.05. The overall level of practice of key messages for optimal breastfeeding was 63.9% (95% CI: 59.1-68.2). The odds of practicing key messages for optimal breastfeeding were higher for mothers who attended antenatal care four times or more (AOR = 2.7, 95% CI: 1.4-5.3), received counseling on breastfeeding during postnatal care (AOR = 3.7, 95% CI: 2.2-6.4), had a good knowledge of breastfeeding (AOR = 6.2, 95% CI: 3.6-10.7), and had a favorable attitude toward breastfeeding (AOR = 6.1, 95% CI: 3.5-10.6). Breastfeeding key message practices among lactating mothers in the study area fell below the recommended universal coverage of 90%. Factors identified included the number of antenatal care visits, counseling on breastfeeding, level of knowledge, and attitude toward breastfeeding. Promoting regular antenatal visits, providing thorough postnatal counseling, and enhancing knowledge and attitudes are crucial for improving optimal breastfeeding practices.