All I want to do is send text to my Arduino and display it on an LCD Screen via Serial
To recap what was done to partially solve the problem:


Verify that lcd.print("Hello, World"); worked OK in setup()
Tried to do print(char); in setup: that worked fine.  The conclusion from this was that something was going wrong in the loop()
Put delay(1000); inside loop(): now the characters showed up correctly on the OLED display.


This suggests that Serial.available() was always returning non-zero for some reason, so that is something that still needs to be looked at.  Most likely the characters coming in from the serial port were spaces since most of the time nothing was visible on the OLED display.