Going by the AP Poll since 2008, Bo Pelini teams are 9 - 16 against teams that were ranked in the Top 25 at game time.
All three games against ranked opponents in 2008 were losses.
The three wins in 2009 (Missouri, Oklahoma and Arizona) were against teams that finished the season unranked.
The two wins in 2010 (Missouri and Oklahoma State) were against teams that finished the season #18 and #13 respectively.
The two wins in 2011 were against Michigan State, which finished the season #11 and Penn State, which finished the season unranked.
The one win in 2012 was against Michigan, which finished the season #24.
The one win in 2013 was against Georgia, which finished the season unranked.
No wins against ranked teams in 2014.
That adds up to a total of four wins in seven years against teams that ended the season in the Top 25:
2010 Okie State #13
2010 Missouri #18
2011 Michigan State #11
2012 Michigan #24