B737-800 Flight Attendant reported incorrect O2 bottles installed on aircraft and also reported that it is a continuing problem.

Narrative

During preflight checks; discovered at least four out of five O2 Walk-Around bottles of incorrect type provisioned on this aircraft. Aircraft was mistakenly fitted with four 66N O2 4.25cu ft bottles instead of the required 4.2 cu ft bottles which supply either only 4L (Hi flow) of O2 per minute; or 4.2 cu ft bottles which supply both 4L (Hi flow) and 2L (Lo Flow). The 66N bottles found onboard only supply 2L (Lo Flow); insufficient to appropriately mitigate a medical or decompression emergency. Aircraft operated unknown multiple cycles with incorrect type of required FAR emergency equipment while not under MEL. Advised Captain; technicians refitted aircraft with correct types of bottles. Aircraft departed without further incident.This identical issue has been previously documented in prior reports. However; never have I discovered such a large error of provisioning of cabin emergency equipment. How could such an egregious breach of basic provisioning of cabin safety equipment occur? Further; this event exposes FAs lack of knowledge and understanding of specifications and correct operation of the various types of O2 equipment found onboard Company's aircraft. Astonished to find such egregious insufficient attention to detail on key pieces of onboard cabin safety equipment. Honestly; have no suggestions to mitigate this risk except a complete audit of provisioning of aircraft cabin emergency equipment. Where is the necessary oversight to prevent such a significant error?