Doyle was in the saddle each time Al Sagar's star mare Nashwa took to the track, guiding her to three Group One victories at the peak of her career.
The formal retainer may have now come to an end, but Al Sagar and his racing manager Teddy Grimthorpe hope to maintain their link with the rider.
The latter said: "We're keen to stress that we have had a fantastic relationship with Hollie throughout, and we hope that will continue for many years to come in different forms.
"There are, however, moments when it is time for a change and Imad felt this was the right time to move on.
"We will now use Oisin, when available, in the absence of Hollie."