The recently published Kepler mission Data Release 25 (DR25) reported on ~197000 targets observed during the mission. Despite this, no wide search for red giants showing solar-like oscillations have been made across all stars observed in Kepler's long-cadence mode. In this work, we perform this task using custom apertures on the Kepler pixel files and detect oscillations in 21914 stars, representing the largest sample of solar-like oscillating stars to date. We measure their frequency at maximum power, {nu}_max_, down to {nu}_max_~=4{mu}Hz and obtain log(g) estimates with a typical uncertainty below 0.05dex, which is superior to typical measurements from spectroscopy. Additionally, the {nu}_max_ distribution of our detections show good agreement with results from a simulated model of the Milky Way, with a ratio of observed to predicted stars of 0.992 for stars with 10<{nu}_max_<270{mu}Hz. Among our red giant detections, we find 909 to be dwarf/subgiant stars whose flux signal is polluted by a neighbouring giant as a result of using larger photometric apertures than those used by the NASA Kepler science processing pipeline. We further find that only 293 of the polluting giants are known Kepler targets. The remainder comprises over 600 newly identified oscillating red giants, with many expected to belong to the Galactic halo, serendipitously falling within the Kepler pixel files of targeted stars.