To also add, you're upgrading from the i5 8400 65w TDP CPU.
It will be easier to upgrade to another 65w CPU if you plan on keeping the stock heat sink.
Otherwise if you plan on the overclockable K version, you will want to upgrade the heat sink.
HP only installs 120mm AIO (designed by Asetek specifically for HP) on all of its 95w K version CPU's. This means you would have to take everything out and pull the motherboard, to install a liquid cooler. HP does not sell those coolers separately either. You would have to look for a single fan third party cooler yourself.
Some places on the web sell used AIOs that came from the Omen...
part number: 928212-001 Thermal Module - Intel Water 95W
http://h10032.www1.hp.com/ctg/Manual/c05827113