Here is an alternative approach that computes at least ##\dim H^0(O) - \dim H^1(O) = \chi(O)##, and without knowing the value of the cohomology groups of the projective plane. This reduces the problem to showing ##\dim H^0(O) = 1##, i.e. the only global sections of ##O## are constants. I learned this idea from a lovely paper of W. Fulton: A note on the arithmetic genus, where he cites Severi (1909) and Zariski (1952), a nice instance of cohomological insights preceding the introduction of cohomology!
To show that ##\chi(O) = 1 - (d-1)(d-2)/2 = 1-g##, we will compute how ##\chi(O)## varies as a curve moves in a linear series.
Lemma A: If ##X,Y## are two curves on a smooth surface ##S##, and if ##X, Y## are linearly equivalent as divisors on ##S##, then ##\chi(O_X) = \chi(O_Y)##.
Remark: This says in some sense ##\chi(O)## is a deformation invariant, at least for linear deformations.
Proof: Since the line bundles ##O_S(-X)## and ##O_S(-Y)## are isomorphic on ##S##, the invariants ##\chi(O_S(-X))## and ##\chi(O_S(-Y))## are equal. By the usual exact sheaf sequence
## 0 \to O_S(-X) \to O_S \to O_X \to0##, and the analogous one for ##Y##, plus the additivity of ##\chi## we get that ##\chi(O_X) = \chi(O_S) - \chi(O_S(-X)) = \chi(O_S) - \chi(O_S(-Y)) = \chi(O_Y)##. qed.
Thus if the formula holds for one curve of degree ##d##, it holds for all. Thus we may compute it for a smooth curve, or for a reducible curve.
Lemma B: Now suppose that ##Y, Y'## are curves on a smooth surface ##S##, and that ##Y## and ##Y'## meet transversely at precisely ##n## points. Then we claim ##\chi(O_{Y+Y’}) = \chi(O_Y) + \chi(O_Y’) - n##.
Proof: Consider the sequence ##0\to O_{Y+Y’} \to O_Y + O_Y’\to O_{Y.Y'}\to 0##, induced by the map from the disjoint union of ##Y,Y'##, to their union ##Y+Y'## on ##S##, and where the map to ##O_{Y.Y’}## is the difference of the two restrictions, from ##Y## and from ##Y'##, to the intersection ##Y.Y'##, of ##Y## and ##Y'##. The additivity of ##\chi## then implies the desired relation, i.e. $$\chi(O_Y) + \chi(O_Y’) = \chi(O_Y + O_Y’) = \chi(O_{Y+Y’}) + \chi(O_{Y.Y’}) = \chi(O_{Y+Y’}) + n$$ Thus ##\chi(O_{Y+Y’}) = \chi(O_Y) + \chi(O_Y') - n##. qed.
[Remark: If we combine lemma B with (the proof of) lemma A, we get a formula for the intersection number of two curves on a surface, in terms of invariants of the surface and the curves, i.e. Bezout's theorem.]
Corollary: If ##X## is a smooth plane curve of degree ##d##, then ##\chi (O_X) = 1 - (d-1)(d-2)/2##.
Proof: Induction on ##d##. If ##d = 2##, then the smooth conic ##X## moves in a linear series also containing a union ##Y## of two lines ##Y_1 + Y_2##, where each line is isomorphic to ##X##. Then by lemmas A, B above, we have ##\chi(X) = \chi(Y_1)+\chi(Y_2) - 1= \chi(X)+\chi(X)-1##, hence ##\chi(X) =1##. This proves the case ##d = 2##, and since a smooth curve of degree ##d = 1## is isomorphic to one of degree ##2##, we also obtain the formula for degree ##d=1##.
Now assume ##d ≥ 3## and that we have proved the formula for smooth curves of degree ##< d##. A smooth degree ##d## curve ##X## moves in a linear series that also contains a curve of form ##Y= Y_1+Y_2##, where ##Y_1## is smooth of degree ##d-1##, and ##Y_2## is a line meeting ##Y_1## transversely in ##d-1## distinct points. Then lemmas A, B and induction give that $$\chi(O_X) = \chi(O_Y) = \chi(O_{Y_1})+\chi(O_{Y_2})-(d-1) = 1-(d-2)(d-3)/2 + 1 - (d-1) = 1-(d-1)(d-2)/2$$ as desired.
q e d .
Note also, that over the complex numbers, a line and a conic are both homeomorphic to a sphere, hence have topological genus zero, so adding a transverse line to a smooth curve of degree ##d##, and smoothing out the intersection points, adds ##(d-1)## to the topological genus. Hence by the same induction argument, the formula ##(1/2)(d-1)(d-2)## is also the topological genus g of a smooth curve of degree d.
Since sheaf sequences make it trivial to show (by induction) that ##\chi(D) - \chi(O) = \deg(D)##, for any divisor ##D## on a curve, these two calculations together prove the weak Riemann Roch theorem for a plane curve: ##\chi(D) = \deg(D) + \chi(O) = \deg(D) + 1-g##.