How can I improve my Low Lighting Shots

[FredS]

I have an Olympus E-520 and mainly use a a 70-300 lense.
I know Olympus is not the best for low light.
Without buying Expensive Glass
The bird was shot at ISO 800 shutter 1250 A Sunny Day.
( I forgot to re set my ISO from the previous day )
The Squirrel was shot ISO Shutter 250 Very cloudy day
Both at 300 mm and croped 100 %
IMO they are OK , just looking for improvments mainly in IQ. at ISO 100 on a sunny day they are much better.
Thanks in advance
Fred..