We present a study of bar and host disk evolution in a dense cluster environment, based on a sample of similar to 800 bright (M-V <= -18) galaxies in the Abell 901/2 supercluster at z similar to 0.165. We use Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) F606W imaging from the STAGES survey, and data from Spitzer, XMM-Newton, and COMBO-17. We identify and characterize bars through ellipse-fitting, and other morphological features through visual classification. We find the following results. (1) To define the optical fraction of barred disk galaxies, we explore three commonly used methods for selecting disk galaxies. We find 625, 485, and 353 disk galaxies, respectively, via visual classification, a single component Sersic cut (n <= 2.5), and a blue-cloud cut. In cluster environments, the latter two methods suffer from serious limitations, and miss 31% and 51%, respectively, of visually identified disks, particularly the many red, bulge-dominated disk galaxies in clusters. (2) For moderately inclined disks, the three methods of disk selection, however, yield a similar global optical bar fraction (f(bar-opt)) of 34%(+10%)(-3%) (115/340), 31%(+10%)(-3%) (58/189), and 30%(+10%)(-3%) (72/241), respectively. (3) We explore f(bar-opt) as a function of host galaxy properties and find that it rises in brighter galaxies and those which appear to have no significant bulge component. Within a given absolute magnitude bin, f(bar-opt) is higher in visually selected disk galaxies that have no bulge as opposed to those with bulges. Conversely, for a given visual morphological class, f(bar-opt) rises at higher luminosities. Both results are similar to trends found in the field. (4) For bright early-types, as well as faint late-type systems with no evident bulge, the optical bar fraction in the Abell 901/2 clusters is comparable within a factor of 1.1-1.4 to that of field galaxies at lower redshifts (z < 0.04). (5) Between the core and the virial radius of the cluster (R similar to 0.25-1.2 Mpc) at intermediate environmental densities (log(Sigma(10)) similar to 1.7-2.3), the optical bar fraction does not appear to depend strongly on the local environment density tracers (kappa, Sigma(10), and intracluster medium (ICM) density), and varies at most by a factor of similar to 1.3. Inside the cluster core, we are limited by number statistics, projection effects, and different trends from different indicators, but overall f(bar-opt) does not show evidence for a variation larger than a factor of 1.5. We discuss the implications of our results for the evolution of bars and disks in dense environments.