A downhole assembly including a rotary steerable tool and a high frequency torsional oscillation mitigation tool (20; 120; 320; 520; 620), the high frequency torsional oscillation mitigation tool being located above the rotary steerable tool, the high frequency torsional oscillation mitigation tool having: a first end and a second end, a first connector (142; 542; 642) at the first end and a second connector (144; 344; 544) at the second end, the first and second connectors being configured for releasable connection to other downhole components in use, characterised in that the high frequency torsional oscillation mitigation tool has a one-way coupling (100; 300; 500; 600) between the first end and the second end, the one-way coupling having a first part and a second part, the first part being connected to rotate with the first connector and the second part being connected to rotate with the second connector, the one-way coupling having an engaged condition in which the first part and the second part can rotate together and a disengaged condition in which the first part can rotate relative to the second part, in that the first part is a tubular shaft (134; 334; 534; 634) and the second part is a sleeve (140; 340; 540; 640) surrounding the tubular shaft, in that one of the sleeve and tubular shaft carries a number of rollers (130; 530) of circular cross-section, each of the rollers being located in a recess (132; 532) of the sleeve or tubular shaft respectively, each recess having a floor which is inclined from a radially shallower end to a radially deeper end, and in which the roller projects from the recess when located at the radially shallower end, and in that each of the rollers comprises a set of aligned roller elements. a first end and a second end, a first connector (142; 542; 642) at the first end and a second connector (144; 344; 544) at the second end, the first and second connectors being configured for releasable connection to other downhole components in use, characterised in that the high frequency torsional oscillation mitigation tool has a one-way coupling (100; 300; 500; 600) between the first end and the second end, the one-way coupling having a first part and a second part, the first part being connected to rotate with the first connector and the second part being connected to rotate with the second connector, the one-way coupling having an engaged condition in which the first part and the second part can rotate together and a disengaged condition in which the first part can rotate relative to the second part, in that the first part is a tubular shaft (134; 334; 534; 634) and the second part is a sleeve (140; 340; 540; 640) surrounding the tubular shaft, in that one of the sleeve and tubular shaft carries a number of rollers (130; 530) of circular cross-section, each of the rollers being located in a recess (132; 532) of the sleeve or tubular shaft respectively, each recess having a floor which is inclined from a radially shallower end to a radially deeper end, and in which the roller projects from the recess when located at the radially shallower end, and in that each of the rollers comprises a set of aligned roller elements. <EOS>