An on-body injector for use with a patient with an injection device has an injection port in fluid communication with a delivery tube, the injection port lying on an injection axis, the on-body injector comprising a bolus reservoir； a bolus injection needle in fluid communication with the bolus reservoir, the bolus injection needle having a bolus injection needle tip aligned with the injection port, the bolus injection needle being slideably biased away from the injection port to define a gap between the bolus injection needle tip and the injection port； and a button operably connected to the bolus injection needle to slide the bolus injection needle along the injection axis, The button is operable to advance the bolus injection needle tip to close the gap and advance the bolus injection needle tip into the injection port to form an injection flow path from the bolus reservoir, through the bolus injection needle, through the delivery tube, and into the patient. The button is further operable to advance a plunger through the bolus reservoir to deliver a predetermined bolus volume to the patient through the injection flow path. The plunger being arranged to block the injection flow path when the plunger is at a final position in the bolus reservoir when the predetermined bolus volume has been delivered.
