  We propose a scenario for the quasi-periodic oscillations observed in
magnetar flares wherein a tangled component of the stellar magnetic field
introduces nearly isotropic stress that gives the fluid core of the star an
effective shear modulus. In a simple, illustrative model of constant density,
the tangled field eliminates the problematic Alfv\'en continuum that would
exist in the stellar core for an organized field. For a tangled field energy
density comparable to that inferred from the measured dipole fields of $\sim
10^{15}$ G in SGRs 1806-20 and 1900+14, torsional modes exist with fundamental
frequencies of about 20 Hz, and mode spacings of $\sim 10$ Hz. For fixed
stellar mass and radius, the model has only one free parameter, and can account
for {\em every} observed QPO under 160 Hz to within 3 Hz for both SGRs 1806-20
and 1900+14. The combined effects of stratification and crust stresses
generally decrease the frequencies of torsional oscillations by $<10$% for
overtones and increase the lowest-frequency fundamentals by up to 50%, and so
the star can be treated as having constant density to a generally good first
approximation. We address the issue of mode excitation by sudden readjustment
of the stellar magnetosphere. While the total energy in excited modes is well
within the energy budget of giant flares, the surface amplitude is $< 10^{-3}$
of the stellar radius for global oscillations, and decreases strongly with mode
frequency. The 626 Hz QPO reported for SGR 1806-20 is particularly problematic
to excite beyond a surface amplitude of $10^{-6}$ of the stellar radius.
