  We have performed a frequency analysis of 10,092 Delta Scuti-type stars
detected in the fourth phase of the Optical Gravitational Lensing Experiment
(OGLE) towards the Galactic bulge, which is the most numerous homogeneous
sample of Delta Scuti stars observed so far. The main goal was to search for
stars pulsating in at least two radial modes simultaneously. We have found 3083
candidates for such stars, which is the largest set obtained to date. Among
them, 2655 stars pulsate in two radial modes, 414 stars pulsate in three radial
modes, and 14 stars pulsate in four radial modes at the same time. We report
the identification of 221 Delta Scuti stars pulsating in the fundamental mode,
first overtone, and third overtone simultaneously. We show the most populated
Petersen and Bailey diagrams and discuss statistical properties of the
identified frequencies based on this numerous sample. Additionally, we present
theoretical predictions of period ratios for Delta Scuti stars pulsating in
overtones from the fourth to the seventh.
