This study aimed to investigate the effect of oral and written contextualization of collocation teaching on the learning and long-term retention of semantically semitransparent collocations by Iranian EFL learners. To this end, 65 students were selected from a cohort of students who were studying English at an English language institute in Sanandaj, Iran. They were randomly divided into two groups based on whether they would receive oral and written contextualization. Then, each group was further subdivided into two groups based on whether they would receive instruction on the concept of semantic transparency, yielding two experimental groups each including 16 learners as the Written Practice Group plus and minus instruction on the concept of semantic transparency, two experimental groups as the Oral Practice Group plus and minus instruction on the concept of semantic transparency, including 16 and 17 learners, respectively. Their initial general English proficiency was measured through the Preliminary English Test (PET), and they were pretested on semitransparent collocations through a researcher-made collocation test. Finally, two equivalent researcher-made collocation tests were used to measure the participants’ learning and retention on the immediate and delayed posttests. The results of two-way ANOVAs revealed that the oral group outperformed the written group on the immediate and delayed posttests and the plus transparency instruction groups in each modality (oral and written) group outperformed the minus transparency instruction groups. The interpretations of the findings and their pedagogical implications have been discussed.