A 94 V-6 Camry with 220,000 miles shifts perfectly except for going into overdrive. Cold or hot it is the same. Someone suggested speed sensor, another suggested shift selenoid. Two even saidreplace thermostat.

Check engine light is not coming on. The button on the shifter is functioning properly it seems with light on dash indicating O/D on or off.

Any suggestions?

 

All good suggestions. Rather than the thermostat (if your heat gauge is showing normal temp) I would look to the engine coolant temp sensor. The O/D engagement is delayed until engine reaches set temp. I'm not sure about speed sensor if all your other shifting is OK. A solenoid is possible, maybe just a broken wire or bad connection. The solenoids are not hard to reach. There will be no code if the solenoid is mechanically not working. When was the last time the trans oil was replaced? Here is a link to the online manual to help you troubleshoot.
http://www.camrymanuals.com/