It is a myth that PADI OWD are strictly only allowed to dive to 18m.
It is recommended that if diving without direct supervision they should stay within their area of experience. Since within the course 18m is the limit, any dive deeper than that would at least require an introduction into deep diving (defined as anything deeper than 18m), as it would be when diving from a boat, in current, in fresh water etc. for the first time. When diving under unfamiliar conditions you always need at least a qualified introduction - which must not necessarily be a course!
This is also quite logical:
Nobody can force you to do another course after the OWD. After 10 years of diving you might have done 300 or 400 dives, still being an OWD, and why should you not dive deeper than 18m, when anybody who does the AOWD right after the OWD, having a total of 9 dives, should be allowed to do so.
Under direct supervision with a qualified guide you are certainly allowed to go deeper than 18m, since this is simply an introduction into a new diving environment.
Dive centers who claim otherwise are only after your money by forcing you to enroll in a deep diving specialty or the AOWD.