I experienced a mild 1 or 2 hz. lateral (yaw) oscillation during a reduced-thrust TO in a TriStar. The captain suspected a FCU instability in #3 RB211 engine; I expressed my doubts - the frequency was too high to be induced by an engine.
On the next TO we used full rated thrust, with no oscillation noted. Maybe it was the FCU after all - but only at reduced thrust TO.