Is there any difference between the Ok() method new ObjectResult()?

Update: Both approaches in the original question + the third approach in the accepted answer has now been superseded by simply returning the object directly:

 return myResult

Relevant example and explanation from the current tutorial page:

[HttpGet("{id}")]
public async Task<ActionResult<TodoItem>> GetTodoItem(long id)
{
    var todoItem = await _context.TodoItems.FindAsync(id);    
    if (todoItem == null)
    {
        return NotFound();
    }    
    return todoItem;
}

The return type of the GetTodoItems and GetTodoItem methods is ActionResult< T > type. ASP.NET Core automatically serializes the object to JSON and writes the JSON into the body of the response message. The response code for this return type is 200, assuming there are no unhandled exceptions. Unhandled exceptions are translated into 5xx errors.

Technically there is no difference between the two approaches.

If you want to look at the code of OkObjectResult then you will see that the OkObjectResult is an ObjectResult that sets the 200 status code, which is the default of ObjectResult already.

The only difference for me is readability in code and your own or your team preferences. It is all about naming and what intention you want to stress.

 return Ok(myResult);                  // gives emphasis on status, my personal favorite

 return new OkObjectResult(myResult);  // for me a little bit verbose and the same
                                       // effect as Ok; but states we return an Object

 return new ObjectResult(myResult);    // gives emphasis of the content that is returned

I can only see the difference in relying on some default value somewhere and providing this value explicitly - the latter is usually better and your intention is much more clear with OkObjectResult (or setting StatusCode explicitly), which is important enough. ObjectResult does not have default StatusCode of 200 - it actually have this value null by default. However, HttpResponse has default status code 200, so it works the same anyway.