Maybe your cup or gpu is overheating after 2hrs of gaming. I personally don’t recommend oppo as their after sales service is quite bad. They didn’t want to repair my camera on my r9s that has focus issues I didn’t drop my phone tho. Just restart your device and try not to play for very long. If it gets worst, like keeps making the noise and the whole phone lags, go to oppo service center and get a replacement of the motherboard. Mistakenly your gpu and cup freezes under high load as the gpu and cpu is not that good compared to other mid range phones like the RedMI note series. Mediatek sucks, you should get a snapdragon chipset phone as Mediatek has software update issues and also overheating issues.
Hope this helps you