§ 91.159
VFR cruising altitude or
flight level.
Except while holding in a holding pattern of 2 minutes or less, or while turning, each
person operating an
aircraft under
VFR in level cruising flight more than 3,000 feet above the surface shall maintain the appropriate altitude or
flight level prescribed below,
unless otherwise authorized by ATC:
“OK” is not in the Pilot /Controller Glossary. If the controller issued “Maintain 5000”, it would be legal
. Good luck finding a controller who will assign you an IFR altitude at your request in Class E.