This article will cover what is different OSS Vendor form Stock Vendor, all the pro and cons
Stock-vendor refers to the firmware for the phone that is developed by the manufacturer of the phone.
ussually its more to be more stable than the OSS-vendor.
The problem with the stock-vendor is that in time the support will stop (no more upgrades to newer Android versions), because the manufacturer prefers you to buy a new phone instead of using your phone for years.
The OSS-vendor refers to a firmware developed by the open source community. It is expected that the OSS-vendor will be supported for much longer than the stock-vendor.
Some or maybe most ROM-developers plan to switch to OSS-vendor once it is stable enough.