Inside an O'Neill cylinder, would a Pendulum's swing have any notable characteristics? Objects inside an O'Neill Cylinder would feel the Centrifugal effect, given by: $F=mω^2r$, and the Coriolis effect, given by: $F=d(mv)/dt$, as well as air friction from air under the same effects.
At the two extremes of the swing, the pendulum would be (at least slightly) closer to the center of the cylinder's axis, and thus experience less "gravity" from the Centrifugal effect. /
Unless perpendicular to the direction of spin, the pendulum would be going against the Coriolis effect in one direction of the swing, and be aided by it in the return direction.
In layman's terms, what would be the observed results of these conflicting forces on a swinging pendulum?
 A: Not noticeable.
On Earth, gravity is slightly weaker at the extreme ends of a pendulum's swing as well, because it's farther away from the center of the Earth. But, it turns out that deviation is small enough that purely geometric effects invalidate the harmonic oscillator approximation for large swing angles long before gravity becomes an issue.
The whole point of an O'Neill cylinder is to be big enough that the gravity feels normal to humans. Thus, while the drop-off of apparent gravity with height on an O'Neill cylinder follows a different equation than gravity on Earth, within the range of angles for which pendulums behave approximately harmonically, it won't make any observable difference. At large angles, it will become shaky and irregular, in ways that are chaotically different from how they would be on Earth... but are chaotically different from how they are on Earth, so no human observer would be able to pick out the differences.
Meanwhile, the tangential velocity at the inner surface of an O'Neill cylinder is just over 195m/s. Your pendulum will not move at any significant fraction of that speed, so the difference in effective gravity on the foreswing vs. the backswing will also be below the margin of error for any human observer.
