Parabola y2=4x has vertex O=(0,0). By symmetry, let A=(t2,2t), B=(t2,−2t).
OA=tt2+4, AB=4t.
For equilateral: OA=AB⇒tt2+4=4t⇒t2=12⇒t=23.
A=(12,43), B=(12,−43).
Circle with AB as diameter: centre (12,0), radius =43.
Minimum distance from origin =12−43=4(3−3).