  We present the first coordinated VLBA/VLTI measurements of the stellar
diameter and circumstellar atmosphere of a Mira variable star. Observations of
the v=1, J=1-0 (43.1 GHz) and v=2, J=1-0 (42.8 GHz) SiO maser emission toward
the Mira variable S Ori were conducted using the VLBA. Coordinated
near-infrared K-band measurements of the stellar diameter were performed using
VLTI/VINCI closely spaced in time to the VLBA observations. Analysis of the SiO
maser data recorded at a visual variability phase 0.73 show the average
distance of the masers from the center of the distribution to be 9.4 mas for
the v=1 masers and 8.8 mas for the v=2 masers. The velocity structure of the
SiO masers appears to be random with no significant indication of global
expansion/infall or rotation. The determined near-infrared, K-band, uniform
disk (UD) diameters decreased from ~10.5 mas at phase 0.80 to ~10.2 mas at
phase 0.95. For the epoch of our VLBA measurements, an extrapolated UD diameter
of Theta_{UD}^K=10.8 +/- 0.3 mas was obtained, corresponding to a linear radius
R_{UD}^K = 2.3 +/- 0.5 AU or 490 +/- 115 solar radii. Our coordinated VLBA/VLTI
measurements show that the masers lie relatively close to the stellar
photosphere at a distance of ~2 photospheric radii, consistent with model
estimates. This result is virtually free of the usual uncertainty inherent in
comparing observations of variable stars widely separated in time and stellar
phase.

Astrophysics