Standard fastboot oem unlock does not work.
Since dedicated ROMs don't exist, your only real option for changing the software on the OPPO A11K is a GSI (Generic System Image) . Android Project Treble allows a generic Android image to boot on any Treble-compatible device. oppo a11k custom rom link
For OPPO A11K:
The Oppo A11K supports Project Treble. This means you can flash any ARM64 A/B GSI ROM. Popular GSIs include: Standard fastboot oem unlock does not work
GSI Download Link: Treble GSIs for Oppo A11K (ARM64 AB) GSI Download Link: Treble GSIs for Oppo A11K (ARM64 AB)
Before flashing anything, you must understand the risks:
If you are ready to risk your device, follow this guide precisely. This is the only working method for the oppo a11k custom rom link situation.