This doctoral research investigates the effect of the use of the Oxford Online Collocation Dictionary on Vietnamese advanced learners' collocation use in academic writing and their perceptions of the use of the dictionary as a supportive tool. The study aims to help learners improve their collocation use, especially advanced learners who are expected to enhance their store of vocabulary on their own. This study analysed students' written texts, questionnaires, observations, and interviews. It was carried out in two phases. In phase 1, 29 participants' 350-word essays on an assigned topic were collected as baseline data. Participants were then instructed in the use of the Oxford Online Collocation Dictionary. In phase 2, a second set of essays were collected. Observations and questionnaires were also collected at this stage and 8 participants were chosen for in-depth interviews. Results showed that the use of this online collocation dictionary as a supportive tool produced some benefits for learners, but these were mainly psychological rather than practical effects. Learners feel confident and assured that they have the tool as a resource. However, their collocation use did not show improvement overall. The study found that learners made mistakes with V-N collocation the most. The study also found that learners often used the dictionary while doing the writing, with some references to other dictionaries for collocation meaning-checking. The dictionary was highly evaluated as a tool for collocation check-up but some enhancements, mostly of content, need to be made. In particular, it needs to provide learners with the meaning of each group of collocates, and pronunciation description; more examples are also needed to illustrate how collocations are used.