What altitude to use for IFR off-airways route
This article covers the topic quite nicely

To begin with, a direct route will only be approved in a radar
environment. In many areas of the country this is not an issue. The
pilot, however, must be aware that filing an altitude at or near the
OROCA (Off-Route Obstruction Clearance Altitude) may not guarantee
radar coverage. In that case, the pilot should expect to be assigned a
different altitude or route.

Its also covered in this AOPA article a bit

If you plan a &quot;direct&quot; flight, point-to-point, follow the rules and
enter the correct equipment. For example, if &quot;/G&quot; (or &quot;slash Golf,&quot;
meaning that your airplane is equipped with an IFR-approved GPS and a
Mode C transponder) or &quot;/I&quot; (you have loran or VOR/DME and a Mode C
transponder) are not entered for these kinds of flights, the air
traffic control computer will reject your flight plan because that
route can't be flown with less capable equipment. Direct routing
requests also are approved only in a radar environment, so forget
about filing direct, off-airways when radar is not available, or in
areas of poor radar coverage.

So broadly speaking you can file the OROCA but ATC may issue a higher altitude for radar coverage reasons. I have also received direct routings en-route at previously filed altitudes if they meet the requirements. Where I fly its common for ATC to prefer an airway route to get you out of the terminal area but will generally clear you direct once in route.