The study investigated parental emotional attachment, birth order and sexual orientation. These were with a view to providing informationon factors that could contribute to sexual orientation among undergraduates in selected universities in Osun State. The study adopted survey research design. The sample size was 900 undergraduates who were selected using multi-stage sampling procedure. Three universities were selected using purposive sampling technique; using ownership as basis for selection (one Federal, one State and one Private). Thirteen faculties were selected across three selected universities by proportionate stratified sampling technique (seven faculties from federal, four from state and two from private universities). Three hundred students were selected from each of the universities using convenience sampling technique. An instrument titled Correlates of Sexual Orientation Questionnaire (CSOQ) was used to collect data from the respondents. Pearson Product Moment Correlation Coefficient and Chi square were employed to analyse the data. It was revealed that there was a positive significant relationship between parental emotional attachment (r = 0.085, p < 0.05) and sexual orientation; it was also revealed that there was a positive significant relationship between birth order (Χ² = 12.11, p < 0.05); and sexual orientation.