Oracle connection on ASP.NET application

Question

Suppose oracle is database. Oracle server and webserver will be hosted on same machine at the time of deployment.
3 queries:
1. Suppose windows based application in C#. Say 5 users. Do we need to install oracle client on each user to connect to Oracle server?
Moreover, what should be the syntax for creating/connecting to oracle in Windows application using C#?

2. Suppose webbased ASP.NET application. Say 10 users.  Do we need to install oracle client on each user to connect to Oracle server? I think no because web based application user should not require Oracle client on his system. Plz confirm.

Moreover, what should be the syntax for creating/connecting to oracle in ASP.NET application using C#? Keep in mind that Oracle server is just present. On users machines, oracle client is not present.

3. Today we know we are using oracle. tomorrow we might have to switch over to SQL server.
In that case, which namespace, data provider (like odbc,oledb) should be used to make it scalable to SQL server?