Mechanical timepiece oscillator which is isochronous in all positions

A mechanical timepiece oscillator including at least one base arranged to be fixed to a bottom plate or a bridge of a timepiece movement, and at least one inertia element arranged to oscillate about a virtual pivot axis of fixed position with respect to said at least one base or of fixed position with respect to said bases when said oscillator has several, in a plane of pivoting perpendicular to said virtual pivot axis, each said inertia element being suspended to at least one said base by several flexible connections each including at least one elastic strip, and said flexible connections together defining said virtual pivot axis, wherein at least one said flexible connection includes at least one deformable compass-like member comprising said elastic strip forming a first arm arranged, at a first end, to be fixed to said base or integral with said base, angularly movable, in projection onto said plane of pivoting, with respect to another said elastic strip forming a second arm of said deformable compass-like member which, at a second end, is arranged to be fixed to said inertial element, or integral with said inertia element, said first arm and said second arm being joined at a reversal edge defining a virtual apex of said deformable compass-like member, a straight line forming a compass axis joining said virtual pivot axis and the projection of said virtual apex onto said plane of pivoting, and wherein, in an unstressed rest state of said oscillator, the projection of said virtual apex onto a plane defined by said virtual pivot axis and said compass axis is on a first side of said virtual pivot axis, opposite to a second side where said first end and said second end are projected.