Online knowledge sharing is the backbone of organisational competitiveness. It helps organisations survive fierce competition since it often facilitates the transfer of individual knowledge to organisational capital and decreases redundant learning time. Yet, a key issue for organisations is engaging employee participation in online knowledge sharing, as the majority of employees have a tendency to lurk preferring to read the knowledge that is shared without contributing themselves. In the online knowledge sharing literature, rather than examining the driving forces of lurkers, previous studies typically investigate undifferentiated general participants or posters who share knowledge. To date, there is little understanding of the key driving factors of online knowledge sharing behaviour in organisations and the impact of these factors on organisational outcomes, which differs between lurkers and posters in a developing country such as Vietnam. While many Vietnamese companies have the necessary infrastructure for online knowledge sharing, the number of lurkers makes up the majority of online participants, leading to many negative consequences. Thus, the current priority of Vietnamese companies is to identify the driving forces of online knowledge sharing behaviour with the objective of encouraging employees, especially lurkers, to increase employee knowledge exchange and job performance. Based on the online knowledge sharing literature, three key contributing factors of knowledge self-efficacy, perceived ease of use and organisational rewards were identified. The focus of this study is to examine the impact of these factors on knowledge sharing reciprocity and job performance as well as the mediating role of knowledge sharing reciprocity. The differences in the driving forces between lurkers and posters were also assessed. An online survey was conducted, and 792 questionnaires were collected from employees in Vietnamese organisations who had experience with organisational online knowledge sharing. Partial least squares structural equation modelling was used for data analysis. The study has several significant findings. First, knowledge self-efficacy and perceived ease of use are two strong predictors of knowledge sharing reciprocity and job performance regardless of whether participants are lurkers or posters. The increase in confidence that knowledge shared would be valuable to others and using online platforms tends to lead to active participation in online knowledge sharing to enhance mutual knowledge exchange and job performance. Second, lurkers are sensitive to organisational rewards, but posters are not. The provision of organisational rewards such as bonuses, promotions, or job security motivates lurkers to actively participate in online knowledge sharing, leading to improvement in knowledge sharing reciprocity and job performance. Third, knowledge sharing reciprocity mediates the impact of knowledge self-efficacy, perceived ease of use and organisational rewards on job performance in both lurker and poster groups. The increase of expectation in the mutual give and take of knowledge sharing will create a favourable knowledge sharing environment which maintains the knowledge sharing process. Such an environment helps to connect the key influences of knowledge self-efficacy, perceived ease of use and organisational rewards with job performance. The findings of this study have important implications for both practitioners and academics. This research has provided an insight into the differences in lurkers’ and posters’ driving forces in the organisational online knowledge sharing process in a developing and transitional economy such as Vietnam.