  In plasmas where the thermal energy density exceeds the magnetic energy
density ($\beta_\parallel > 1$), the aperiodic ordinary mode (O-mode)
instability is driven by an excess of parallel temperature $A = T_\perp
/T_\parallel < 1$ (where $\parallel$ and $\perp$ denote directions relative to
the uniform magnetic field). When stimulated by parallel plasma streams the
instability conditions extend to low beta states, i.e., $\beta_\parallel <1$,
and recent studies have proven the existence of a new regime, where the
anisotropy threshold decreases steeply with lowering $\beta_\parallel \to 0$ if
the streaming velocity is sufficiently high. However, the occurrence of this
instability is questionable especially in the low-beta plasmas, where the
electrostatic two-stream instabilities are expected to develop much faster in
the process of relaxation of the counterstreams. It is therefore proposed here
to identify the instability conditions for the O-mode below those required for
the onset of the electrostatic instability. An hierarchy of these two
instabilities is established for both the low $\beta_\parallel <1$ and large
$\beta_\parallel > 1$ plasmas. The conditions where the O-mode instability can
operate efficiently are markedly constrained by the electrostatic instabilities
especially in the low-beta plasmas.
