Reentry drilling operations have been one of the most popular means for field development over the past decade. Many of the wells in mature fields tend to suffer from low productivity which would require reentry operations to increase reservoir contact and extend the life of the well. This has been preferable, in many cases, to drilling a completely new well from surface with all the associated costs and risks present in crowded old fields.Reentry drilling operations would normally require slimming down of the well design or the use of newly introduced expandable technologies to maintain similar hole sizes. The use of expandable technologies requires hole enlargement before running the liner. Running and expanding a liner string requires a smooth wellbore with minimal doglegs and tortuosity. A bad quality wellbore will lead to challenges that may result in not being able to run the expandable string to bottom, getting stuck or not fully expanding the liner. Past experiences have shown that utilizing a conventional motor and bent sub assembly to drill the 6 1/8-in. hole, and then run a 7-in. or 7 3/8-in. hole opening assembly has proven to be time consuming and may require under-reaming the same section more than once. The result is a borehole with an inconstant hole diameter, excessive doglegs and high tortuosity due to shifting between sliding and rotary mode. This phenomenon is significantly reduced when using a rotary steerable system (RSS) due to the continuous rotation of the string during directional drilling.In an effort to optimize the reentry operations, Saudi Aramco and the service company utilized a highly sophisticated drilling assembly combining hole-opening technology, high-build-rate RSS and measurements-/logging-while-drilling (MWD/LWD) tools. This was the first time such an assembly has been utilized for this particular application worldwide. The goal was to sidetrack, drill, and enlarge the section in one run, producing a well geometry suitable for landing and expanding the solid expandable tubular. The first application drilled and underreamed the 6 1/8-in. X 7 3/8-in. hole from the kick off point to total depth using Schlumberger's high-dogleg RSS system. As a result, the 5 1/2-in. expandable liner was run to bottom, cemented, and expanded successfully.So far, this system has been successfully run twice in Saudi Aramco reentry operations. In both cases, utilizing the combined drilling system has proven to improve wellbore quality and reduce the well delivery time by an average of 4 operating days.