  We have studied the OH masers in the star forming region, W3(OH), with data
obtained from the Very Long Baseline Array (VLBA). The data provide an angular
resolution of $\sim$5 mas, and a velocity resolution of 106 m s$^{-1}$. A novel
analysis procedure allows us to differentiate between broadband temporal
intensity fluctuations introduced by instrumental gain variations plus
interstellar diffractive scintillation, and intrinsic narrowband variations.
Based on this 12.5 hours observation, we are sensitive to variations with time
scales of minutes to hours. We find statistically significant intrinsic
variations with time scales of $\sim$15--20 minutes or slower, based on the
{\it velocity-resolved fluctuation spectra}. These variations are seen
predominantly towards the line shoulders. The peak of the line profile shows
little variation, suggesting that they perhaps exhibit saturated emission. The
associated modulation index of the observed fluctuation varies from
statistically insignificant values at the line center to about unity away from
the line center. Based on light-travel-time considerations, the 20-minute time
scale of intrinsic fluctuations translates to a spatial dimension of $\sim$2--3
AU along the sight-lines. On the other hand, the transverse dimension of the
sources, estimated from their observed angular sizes of about $\sim$3 mas, is
about 6 AU. We argue that these source sizes are intrinsic, and are not
affected by interstellar scatter broadening. The implied peak brightness
temperature of the 1612/1720 maser sources is about $\sim2\times 10^{13}$ K,
and a factor of about five higher for the 1665 line.
