Using OR in linq expression conditionally

I have fliters in my asp.net project and want to add expression to this list with condition:

Expression<Func<MyModel, bool>> func;
var list = new List<Expression<Func<MyModel, bool>>>();

I want to use "OR" condition between elements of list. for example:

if(sth){
   func = p => p.a <= b;
   list.Add(func);
}

How can I do this?