The nut itself should be fine and nuts have lots of useful nutrition that is hard to find in other sources but they are fattening for those same reasons so some care needs to be taken when formulating a diet involving nuts. I've fed acorns to plenty of other animals prior to getting rabbits and did a school project on acorn flour for human consumption. The shell, leaves, bark, etc.. of an oak tree and acorns should not be fed to anything. Since rabbits do not hull out the shell like a squirrel, other rodent or bird would do letting them have access to whole acorns might result in health problems or at least stress on the liver and internal organs that filter the blood leading to poor condition and breeding difficulties. The nut itself contains none of these harsh compounds and should be just as safe as any other nut. If you find squirrel or mouse collected wild nuts you will see they chew a small hole spitting out the shell pieces and then scrape out the nut meat.