Pilot reported an NMAC while taking off from ONZ airport with a non-communicating helicopter that was proceeding directly to the airport to land.

Narrative

Departed ONZ runway 22. Made my call before entering runway 'Aircraft X departing 22; left traffic to the North; Grose Ile' (to pick up IFR SID thru Bravo). On roll out I heard 'Aircraft Y 3 miles South; Grosse Ile'. (No information given on inboard; heading or anything.) After takeoff at about 400 -500ft AGL we saw a Rotorcraft to our right. I immediately leveled off; but did not make an immediate turn as this would cause losing visual on the close aircraft. We were less than 100ft below and about 200 from the aircraft. After; Aircraft Y announced that he was in downwind runway 22. (he traveled West to east in what appeared to be backward 45 degree to downwind). From the sounds of the called; I believe this was a student pilot. Aircraft Y was 'B-lining' directly to the airport or backward 45 (at 500 AGL which is rotorcraft pattern altitude) without joining a traffic pattern. This put him at 500ft AGL direct over the departure end of runway 22!!! I made a radio call to him; with no response. I understand a class G airport does not legally require radio communication; nor under part 91 require a standard traffic pattern. What the pilot did was extremely dangerous. As mentioned; before I believe he was a student or low hour pilot.