Background: Challenges in phonological processing are prevalent in children with developmental speech and/or language disorders and are associated with their language acquisition and later reading skills. A recent systematic review and meta-analysis from the same project of studies with a group design on the effects of interventions on output phonology, found that oral language interventions have a positive effect on output phonology, specifically speech production accuracy. These interventions are expected to impact the phonological representational level as well and, thus, also affect phonological processing skills in this population. However, no prior systematic reviews or meta-analyses have been conducted on this topic. Aims: We conducted a systematic literature review and meta-analysis to investigate the effectiveness of oral language interventions on children and adolescents with developmental speech and/or language disorders, focusing on phonological processing as the primary outcome measure, including phonological awareness, phonological retrieval efficiency, and short-term memory. Methods and Procedures: The systematic review was pre-registered (PROSPERO ID: CRD42017076075) and followed the PRISMA guidelines. A search for oral language interventions in children and adolescents with developmental speech and/or language disorders (mean age, 3–18 years) was conducted across seven databases (PubMed, Web of Science, ERIC, Cochrane Library, PsycINFO, Scopus, LLBA) for studies published between January 2006 and May 2023. We included studies with a group design and phonological processing as the outcome measure. The Cochrane risk of bias tool was used for study appraisal. Individual and average standardised mean differences g were used in the meta-analysis. We also explored the influence of various moderators. Outcomes and Results: We identified 22 studies that met the inclusion criteria. Overall, the risk of bias was low to moderate. Significant and large effect sizes were found in pre-post studies with control groups, showing improvements post-intervention in phonological awareness, g = 0.710, p < 0.001, 95% CI [0.504, 0.917], and phonological short-term memory, g = 0.785, p = 0.010, 95% CI [0.192, 1.279]. Some moderator variables emerged, for example, larger effects when the interventions were based on an integrated theoretical approach, applied in small groups, or when they had processing targets. Conclusions and Implications: The results demonstrate that oral language interventions can effectively improve specific phonological processing skills in children and adolescents with developmental speech and/or language disorders. The identification of key moderating factors provides valuable guidance for clinical practice. WHAT THIS PAPER ADDS: What is already known on the subject. Children with developmental speech and/or language disorders often have difficulties in phonological processing that can hamper their reading development. Interventions are effective for speech production, but there are no previous systematic reviews or meta-analyses on their effects on the phonological processing skills of children and adolescents with developmental speech and/or language disorders. What this paper adds to existing knowledge. We present empirical evidence of the effectiveness of oral language interventions in improving phonological processing (including phonological awareness and phonological short-term memory) in children and adolescents with developmental speech and/or language disorders. What are the potential or actual clinical implications of this work?. Oral language interventions have a positive effect on the phonological processing skills of children and adolescents with developmental speech and/or language disorders. They may serve as an indirect means of support for their reading development.