  The energies as a function of the magnetic field ($H$) and the pressure are
studied theoretically in the tight-binding model for the two-dimensional
organic conductor,
  $\alpha$-(BEDT-TTF)$_2$I$_3$, in which massless Dirac fermions are realized.
The effects of the uniaxial pressure ($P$) are studied by using the
pressure-dependent hopping parameters. The system is semi-metallic with the
same area of an electron pocket and a hole pocket at $P < 3.0$~kbar, where the
energies $(\varepsilon_{\rm D}^0$) at the Dirac points locate below the Fermi
energy $(\varepsilon_{\rm F}^0$) when $H=0$. We find that at $P=2.3$~kbar the
Dirac cones are critically tilted. In that case a new type of band crossing
occurs at "three-quarter"-Dirac points, i.e., the dispersion is quadratic in
one direction and linear in the other three directions. We obtain new
magnetic-field-dependences of the Landau levels $(\varepsilon_n)$;
$\varepsilon_n-\varepsilon^0_{\textrm{D}} \propto (n H)^{4/5}$ at $P=2.3$~kbar
("three-quarter"-Dirac points) and $|\varepsilon_n-\varepsilon_{\rm F}^0|
\propto (n H)^2$ at $P=3.0$~kbar (the critical pressure for the semi-metallic
state). We also study the magnetization as a function of the inverse magnetic
field. We obtain two types of quantum oscillations. One is the usual de Haas
van Alphen (dHvA) oscillation, and the other is the unusual dHvA-like
oscillation which is seen even in the system without the Fermi surface.
