  We present the results of a detailed analysis of 121 ground-based
high-resolution high S/N spectroscopic measurements spread over 3 years for the
Beta Cephei star Theta Ophiuchi. We discovered Theta Oph to be a triple system.
In addition to the already known Speckle B5 companion of the B2 primary, we
showed the presence of a low-mass spectroscopic companion and we derived an
orbital period of 56.71 days with an eccentricity of 0.1670. After removing the
orbit we determined two frequencies for the primary in the residual radial
velocities: f1 = 7.1160 c/d and f2 = 7.4676 c/d. We also found the presence of
f3 = 7.3696 c/d by means of a two dimensional frequency search across the Si
III 4567 A profiles. We identified the m-value of the main mode with frequency
f1 by taking into account the photometric identifications of the degrees l. By
means of the moment method and the amplitude and phase variations across the
line profile, we derived (l1,m1) = (2,-1). This result allows us to fix the
mode identifications of the whole quintuplet for which three components were
detected in photometry. This is of particular use for our forthcoming seismic
modelling of the primary. We also determined stellar parameters of the primary
by non-local thermodynamic equilibrium hydrogen, helium and silicon line
profile fitting and we obtained Teff = 24000 K and log g = 4.1, which is
consistent with photometrically determined values.
