Ops Manager Install in Windows and connect to mongod instance started in linux
Can we install ops manager in windows and connect to mongodb instance
  started in Linux operating system?


No, I have not done as practically , as per my knowledge due to platform &amp; their connection string used to access the Ops Manager Application Database.


  Does ops manager installation on windows require separate installation
  of mongoDB instance on windows also?


No, Because, the databases require dedicated MongoDB instances. Do not use MongoDB installations that store other data. The Backup Database is required only if you will use the Backup feature.


  Can we install ops manager on linux and access it from windows using
  URL?


Yes,For example you are going to access ops manager through the following URL

http://&lt;OpsManagerHost&gt;:8080


You have to make sure that the host is the fully qualified domain name of the server.

For further your ref here , here and here