How to do RegAsm so that it cover 32-bit and 64-bit?

also, you probably compiled your dll with AllCPU option try to compile with x64 and/or x86 option and register like you do

• for x64 - "C:\Windows\Microsoft.NET\Framework64\v4.0.30319\RegAsm.exe" /register /codebase "C:\x64\o.dll"
• for x86 - "C:\Windows\Microsoft.NET\Framework\v4.0.30319\RegAsm.exe" /register /codebase "C:\x86\o.dll"

P.S. its 5yo question, but may be help to other peoples

You have it backwards. The Wow6432Node is where the 32-bit specific data goes. Use the 32-bit regasm (from the Framework directory, not the Framework64 directory) to have your component registered in the Wow6432Node.