You certainly can read the information from an Excel file either via ActiveX or via ADO. There's a "Write Table to XL" example that ships with LabVIEW that shows the ActiveX way. Basically all you would need is to replace the "Set Cell Value" VI with a "Get Cell Value", which is pretty trivial. The main issue with this method isn't being able to read the Excel file, but rather running into the problem of two applications trying to access the same file at the same time. You will likely run into a file I/O error. Does the oximeter application keep the Excel file open?
I would say that reading the information over the serial port is your best bet since you don't have to deal with ActiveX or ADO, and you're simply using VISA to read information over the serial port. If all you need to do is read three bytes this is trivial. Check out the examples that ship with LabVIEW on reading/writing with the serial port.