Any OS will boot with the CPU clock frequency and voltage that was set up by the firmware/bootloader. Those are supposed to be "safe" settings in the sense that the OS can run without overheating the SoC.
If the DT provides a set of operating points, and there is clock and regulator support, OpenBSD will allow changing the clock frequency. For each frequency it will select the associated voltage in the operating points table. When there is support for changing the clock frequency, OpenBSD will switch to the operating point that has an associated clock frequency that not higher than the clock frequency the system booted with.
