We report the observation of Higgs boson decays to $WW^{\ast}$ based on an excess over background of 6.1 standard deviations in the dilepton final state, where the Standard Model expectation is 5.8 standard deviations. Evidence for the vector-boson fusion (VBF) production process is obtained with a significance of 3.2 standard deviations. The results are obtained from a data sample corresponding to an integrated luminosity of 25~\textrm{pb}^{-1} from $\sqrt{s}=7$ and 8 TeV $pp$ collisions recorded by the ATLAS detector at the LHC. For a Higgs boson mass of 125.36 GeV, the ratio of the measured value to the expected value of the total production cross section times branching fraction is 1.09^{+0.16}_{-0.15}~\textrm{(stat.)}^{+0.17}_{-0.14}~\textrm{(syst.)}. The corresponding ratios for the gluon fusion and vector-boson fusion production mechanisms are 1.02\pm 0.19~\textrm{(stat.)}^{+0.22}_{-0.18}~\textrm{(syst.)} and 1.27^{+0.44}_{-0.40}~\textrm{(stat.)}^{+0.30}_{-0.21}~\textrm{(syst.)}, respectively. At $\sqrt{s}=8$ TeV, the total production cross sections are measured to be \sigma(gg\to~H\rightarrow WW^\ast) = 4.6\pm0.9\,\textrm{(stat.)}\,^{+0.8}_{-0.7}\,\textrm{(syst.)}\textrm{pb} and \sigma(\textrm{VBF}~H\rightarrow WW^\ast) = 0.51\,^{+0.17}_{-0.15}\,\textrm{(stat.)}\,^{+0.13}_{-0.08}\,\textrm{(syst.)}\textrm{pb}. The fiducial cross section is determined for the gluon-fusion process in exclusive final states with zero or one associated jet.

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