The objective of this study is to assess the effects of teacher mediation on students' self efficacy belieD SEB, locus of controll LOC, and oral English perfolmance/ OEP of Addis Ababa preparatory-I students. To this end, an experimental study was carried out. For the experiment, two schools: Bole and Wondrad were selected and 40 students were drawn from each and participated in the contro::~J and mediated oral English lessons, which were conducted by two teachers for four months. Moreover, preparatory-I English teachers were the target of the study in order to collect the data that assisted me to understand the oral English teaching! learning experiences, and played a great role in triangulating the findings. The data were col1ected tlu'ough mixed methods (quantitative and qualitative). The quantitative methods included questionnaires on MLE for both teachers and students, questionnaires on SEB and LOC scales, and OEP for students. The qualitative data were collected tlu'ough class observations, interviews and focus group discussions that were held with both students and teachers. From the quantitative fmdings, it was learned that both teachers and students underlined that MLE criteria could create a good atmosphere to practice and improve oral English. Nevertheless, differences were observed in the teachers' claims to utilize the MLE cliteria, and whal' the students reflected about their oral English teaching 'culture.' The findings of the students' LOC, SEB, and OEP indicated that MLE caused statistically significant changes. These changes were also justified from the comparisons made between mediated and unmediated students. From the qualitative findings, differences were observed in the mediated students' understanding of oral English, the way they evaluated their oral ability, and attributed their success or failure in OEP after the application ofMLE. In order to confiml the occurrence of the changes in the way they used to think about oral English and performance, their responses were compared with that of the unmediated ones, and it was apparent that MLE introduced changes to the mediated students' understanding and reasoning capacity about practices c.hal1p.llges, and possibility to improve their OEP. From the study, It was leamed that the findings have research, pedagogical implications, and recommendations that could initiate future actions in training English teachers in general and oral English teachers in particular have been drawn.