Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Workload; Communication Breakdown; Confusion; Distraction
ATC Issue All Types; Conflict Ground Conflict; Critical
Flight Crew Took Evasive Action
Environment - Non Weather Related; Procedure
While pushing back out of ONT gate onto Taxiway S via Taxiway S2; Ontario Ground Control cleared a single engine aircraft to taxi to parking via Taxiway S2.  Ground Control cautioned the single engine aircraft of a 757 pushing back onto Taxiway S2.  While in the throat of Taxiway S and S2 under the control of the tug with wing walkers; the single engine aircraft taxied past our aircraft on the east side of Taxiway S2.  From our viewpoint in the cockpit the single engine aircraft was outside of the confines of Taxiway S2 on the east side of the taxiway.  The single engine aircraft came very close to our wing walker and the tug. The tug driver came to an emergency stop until the single engine aircraft passed.  We contacted Ontario Ground Control and reported the incident.Poor guidance by Ground Control and poor judgment used by the pilot of the single engine aircraft.Ground Control could have given the single engine aircraft a hold short until we were clear of S2. Ground could have also asked the single engine aircraft to hold short of S2 and asked us to do a long push on S Taxiway and hold until the single engine aircraft had entered the Ramp via S2.
ONT Ground Control cleared a single engine aircraft to park via Taxiway S2 which forced an air carrier tug and wing walkers pushing an aircraft onto S2 to take evasive action until the single engine aircraft passed.