Buck converter without oscillator?
As above, what you have is a linear regulator.
BUT
If you add hysteresis to the circuit, you now have what is called a hysteretic converter, where the output bounces back and forth rapidly between slightly above and slightly below the desired output voltage.  In this case, Q1 is now switching between off and fully enhanced (on) for much lower power dissipation.  So, less heat, but more noise on the output.  This can be done without an inductor, but it increases the stress on the switching transistor.
In its most simple form, a hysteretic converter is a voltage comparator driving the switch transistor.
Way back, the first switching power supply I had to (constantly) maintain was a 20 A hysteretic converter built onto the backplane of a Data General NOVA 1220.  An undersized heatsink and poor airflow caused the transistor to burn up about every 6 months.
AND ...
Gold Star for using reference designators.