  We present a collection of 65,981 Mira-type variable stars found in the
Optical Gravitational Lensing Experiment (OGLE) project database. Two-thirds of
our sample (40,356 objects) are located in the Galactic bulge fields, whereas
25,625 stars are in the Galactic disk. The vast majority of the collection
(47,532 objects) are new discoveries. We provide basic observational parameters
of the Mira variables: equatorial coordinates, pulsation periods, $I$-band and
$V$-band mean magnitudes, $I$-band brightness amplitudes, and identifications
in other catalogs of variable stars. We also provide the $I$-band and $V$-band
time-series photometry collected since 1997 during the OGLE-II, OGLE-III, and
OGLE-IV phases. The classical selection process, i.e., mostly based on the
visual inspection of light curves by experienced astronomers, led us to the
high purity of the catalog. As a result, this collection can be used as a
training set in the machine learning classification algorithms. Using
overlapping parts of adjacent OGLE fields, we estimate the completeness of the
catalog to be about 96%. We compare and discuss the statistical features of
Miras located in different regions of the Milky Way. We show examples of stars
that change their type over time, from a semi-regular variable to Mira and vice
versa. This dataset is perfectly suited to study the three-dimensional
structure of the Milky Way, and it may help to explain the puzzle of the
X-shaped bulge.
