Second language acquisition (SLA) researchers contend that focus on form (FonF) instruction plays a pivotal role in the course of language acquisition. Focus on form (FonF) instruction is learner-centered and is consistent with the learner’s internal syllabus. There has been an ongoing query in the literature as to whether focus on form should occur prior to error commitment or be reactive to the actual errors of the learners during the task. In this regard, this study tried to explore the effectiveness as well as the relative impacts of planned preemptive vs. delayed reactive focus on form on lexical resource of English as a foreign language (EFL) learners’ oral production in meaning-oriented interviews. The study adopted a pretest-posttest quasi-experimental design including two experimental groups, each receiving a distinct treatment during the whole semester. The participants of the study were a total of 19 upper-intermediate adult all-female students. Paired-samples t-test and independent-samples t-test were utilized to evaluate and compare the differing effects of the two treatments respectively. The findings of the study demonstrated that both types of form-focused instructions are conducive to learners’ oral production lexical resource. One of the interesting findings of the present study was that preemptive focus on form can be planned in advance on the basis of teachers’ experience, which can lead to a blurred dichotomy between planned and incidental focus on form.