Disinfection is an action of combatting and removing all microorganisms including spores with physical or chemical procedures. Cross infection is the transmission of pathogenic microorganisms that occurs between dentists and patients, patients and patients, and patients and nurses. The use of disinfectants in dentistry’s instruments is one way to prevent cross infection. Cross infection can occur through microorganisms found in blood, saliva and dental plaque which can contaminate the instruments used by dentistry personnel. Therefore, a transmission of pathogenic microorganisms from instruments that have been contaminated by dentistry personnel occurs through indirect contact. Ozone water and 4.8% chloroxylenol are disinfecting agents used to disinfect medical instruments. This study aims to compare the effectiveness of ozone water disinfection and 4.8% chloroxylenol to the number of bacterial colonies on instruments tooth extraction at the FKG USU Dental and Oral Hospital. This is a laboratory experimental study with a post-test only design group. The sample used is mandibular molar pliers that have been used in tooth extraction procedures. This study used 3 sample groups, where each group consisted of 10 instruments tooth extraction. The treatment group used ozone water and 4.8% chloroxylenol, and the control group used negative distilled water (aquades). The effectiveness test in this study used a bacterial colony counter with a spread plate method. Data were analyzed with the Kruskal Wallis test and follow-up test with Mann Whitney. The results of the data analysis showed a p-value of 0.000, which means that there is a significant ratio of disinfection of ozone water and 4.8% chloroxylenol to the number of bacterial colonies on instruments tooth extraction at the FKG USU Dental and Oral Hospital. The results of this study showed that the average number of bacterial colonies formed in the ozone water treatment group was 4000 ± 1366.26, in the 4.8% chloroxylenol treatment group was 14400 ± 1550.63, and in the negative control group (aquades) was 230200 ± 12182.68. Based on this study, it can be concluded that ozone water is more effective in disinfecting instruments tooth extraction at the FKG USU Dental and Oral Hospital compared to 4.8% chloroxylenol. Aquades is labeled with "A" 4,8% chloroxylenol is labeled with "K" Ozone water is labeled with "Z" The samples are sorted by numbers. Example : A1 meaning is sample aquades no.1

Aquades is labeled with "A" 4,8% chloroxylenol is labeled with "K" Ozone water is labeled with "Z" The samples are sorted by numbers. Example : A1 meaning is sample aquades no.1