An on-body injector and method of use including an on-body injector for use with an injection device. The on-body injector includes 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. 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.