DSolve[eq, y[x], x]DSolve[y''[x] == 0, y[x], x]
DSolve[y''[x] == y[x], y[x], x]
DSolve[y''[x] == y[x], y, x]
DSolve can also solve basic PDE
DSolve[D[f[x, y], x] / f[x, y] + 3 D[f[x, y], y] / f[x, y] == 2, f, {x, y}]
DSolve[D[f[x, y], x] x + D[f[x, y], y] y == 2, f[x, y], {x, y}]
DSolve[D[y[x, t], t] + 2 D[y[x, t], x] == 0, y[x, t], {x, t}]