We have ever heard about singularity the Microsoft project to make an OS in managed language. Most is done in managed mode but some part are done in asm.
Both project have succeed to boot with virtual machine.
Cosmos have made a compiler which convert the IL(intermediate language) code in assembler code. So code is compile just in time but ahead of time. (The mono compiler can ever done it). I do not know if there are differences between mono AOT and Cosmos one…
SharpOS use another way. They build a PE file with MultiBoot header.
If you want to learned how work an OS there is another good project named SOS for Simple OS (fork from KOS) which is lead by a French Linux magazine. This is in C/C++ but you have a lot of documentation which come from articles in the magazine freely available. All article are in french but it can help if you want to help SharpOS or Cosmos and know nothing about kernel.