Introduction: Antimicrobial stewardship (AMS) strategies, such as intravenous-to-oral switch (IVOS), promote optimal antimicrobial use, contributing to safer and more effective patient care and tackling antimicrobial resistance (AMR). Aim: This study aimed to achieve nationwide multidisciplinary expert consensus on antimicrobial IVOS criteria for timely switch in hospitalised adult patients and to design an IVOS decision aid to operationalise agreed IVOS criteria in the hospital setting. Method: A four-step Delphi process was chosen to achieve expert consensus on IVOS criteria and decision aid; it included (Step One) Pilot/1st round questionnaire, (Step Two) Virtual meeting, (Step Three) 2nd round questionnaire and (Step 4) Workshop. This study follows the Appraisal of Guidelines for Research and Evaluation II instrument checklist. Results: The Step One questionnaire of 42 IVOS criteria had 24 respondents, 15 of whom participated in Step Two, in which 37 criteria were accepted for the next step. Step Three had 242 respondents (England n = 195, Northern Ireland n = 18, Scotland n = 18, Wales n = 11); 27 criteria were accepted. Step Four had 48 survey respondents and 33 workshop participants; consensus was achieved for 24 criteria and comments were received on a proposed IVOS decision aid. Research recommendations include the use of evidence-based standardised IVOS criteria. Discussion and Conclusion: This study achieved nationwide expert consensus on antimicrobial IVOS criteria for timely switch in the hospitalised adult population. For criteria operationalisation, an IVOS decision aid was developed. Further research is required to provide clinical validation of the consensus IVOS criteria and to expand this work into the paediatric and international settings.