[DonRotolo]

Voltage regulators have a parameter called "Dropout"- the input voltage at which regulation no longer occurs.

A 78xx series regulator requires an "overhead" - the voltage difference between input and output - of something like 2 or 3 volts to avoid dropout. Low-Dropout (LDO) regulators exist, with overheads of a fraction of a volt.

Your 9 volt transistor battery was very close to the dropout limit for a 7805 regulator already. If you draw more current from the battery tha its rating - I think it's 20 mA for that kind of battery - the battery output voltage can sag (get lower). You may want to check what current the OI draws, either by looking at the specs or by direct measurement, and decide if a two 9 volt batteries in parallel is sufficient to power the OI reliably.

Don