Cisak spent time with the O's last season and made an impression despite their relegation to League Two. He made 19 appearances having previously played for York in the early part of last season.
He made just two appearances for the Clarets. He came on as a substitute when Tom Heaton was sent off at Brighton and stood in for him in the Capital One Cup tie against Preston when Heaton was suspended.
He's signed a two year deal with the London club for an undisclosed fee.
Kirkland opted to leave Hillsborough after losing his place to Keiren Westwood. The 34-year-old has made no fewer than 279 league appearances for seven different clubs and made one appearance for England, coming on as a substitute against Greece in 2006.