Military helicopter instructor pilot reported a light aircraft overflew them by about 300 feet while they were taxiing on the runway at OBI without communicating their intentions.

Narrative

While operating in the day VFR traffic pattern at uncontrolled airport; we observed a single-engine GA aircraft taxiing to the approach end of Runway 31 (R31). We made standard calls on the CTAF frequency; but we did not hear any replies or any calls from the taxiing aircraft. The taxiing aircraft approached the hold short for R31; so we made a call that we would extend our downwind to allow them to depart. We were clearly visible in their field of view in the downwind; but we did not hear a reply. After extending the downwind a mile; and without hearing any calls; we made radio calls and turned base and final for R31. We observed the GA aircraft still holding short and continued our landing down R31. We passed directly in front of the GA aircraft holding short (~200 feet laterally at an altitude of 100' or less). We continued the running landing about halfway down R31 (3;074'x 75'). Since we needed to clean up from a simulated emergency procedure; we taxied further down R31 with the intent to turn off onto R19 to clear the runway for the departing traffic. We made radio calls as we did that but still did not hear any replies from the GA aircraft. As we taxied; we saw the GA aircraft taking off over the top of us and slightly offset; less than 300' above us without making any radio calls. After we cleaned up our simulated EP (Emergency Procedure); we made several radio calls and attempted to call out the aircraft on the radio with no response. The aircraft was apparently not squawking ADS-B; and we could not find him; checking up to 10 minutes later on Foreflight. The aircraft was an small low wing aircraft. We reported the incident to our flight safety department. There are no lessons learned from our end. We verified our radio; TCAS; and ADS-B were functioning properly; and we were making appropriate radio calls. R31 is a short runway with a backdrop of green trees that would easily make the brightly colored helicopter with anticollision lights stand out. The GA pilot apparently rushed his takeoff and assumed we were doing another touch and go even though he could see us still on the runway.