Ireland's O'Driscoll, 34, was expected to be named captain in Sam Warburton's absence, but has been omitted from the squad by head coach Warren Gatland.
Tours: 4 (2001, 2005, 2009, 2013)
Matches: 18 (8 Tests)
Tries: 9 (1 in Tests)
Captain: 7 times (1 Test)
Test record: 2 wins, 6 defeats
"O'Driscoll has been quiet. But he has been the clarion call once [lock] Paul O'Connell got injured," said Wood.
He added on BBC Radio 5 live that the XV had left him "totally at a loss".
Former Ireland and Lions lock Willie John McBride believes Australia coach Robbie Deans will be "laughing" at the "absolutely amazing" decision to drop O'Driscoll.
On BBC Radio 4's World at One programme, he said: "I must admit I am quite gutted today because Brian O'Driscoll in my book has been the greatest player Ireland has ever seen.
"You live and die by your decisions, though, so we will see what happens at the weekend."
The series is currently level at one game apiece after Australia hit back with a gritty 16-15 second Test victory following the Lions' 23-21 win in the opener.
Gatland - who also coaches Wales - has made six changes to the team beaten in Melbourne, with 10 Welshmen set to start in Sydney.
England prop Alex Corbisiero and Ireland flanker Sean O'Brien come into the pack while four Welshmen - fit-again centre Jamie Roberts, hooker Richard Hibbard, scrum-half Mike Phillips and number eight Toby Faletau - complete the changes.
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Lock Alun Wyn Jones captains the side while the 10-strong contingent equals the record for Wales - the previous occasion was the first Test against Australia in 1950 - while England hold the overall record with 12 players in the 1880s.
"You should pick a team with the best players in it and for me the ethos of the Lions is slipping," added McBride.
"I don't think this is the concept of the Lions that I know but let's see what happens on Saturday, we will all eat our words if we win."
The direct approach favoured by Gatland has been dubbed "Warrenball" by sections of the Australian media and Wood - who played on two Lions tours and featured 58 times for Ireland - admitted he was concerned about the tourists' lack of creativity.
The 41-year-old said: "You could say he is picking on form, but he has picked an unbelievably direct team, with very little guile, specifically to play this game-plan.
"You just wonder if they got a lucky bag and picked some names out. The Lions won the first Test and made eight changes to their squad. The Wallabies won the second and probably won't make any changes. The Lions lose by one point and there are six changes in the team and three guys on the bench who were nowhere near the team for the second Test. I was convinced Brian O'Driscoll should have been named as captain. It's catastrophic leaving him out. He's still one of the top centres in world rugby. He's a fantastic guy and has been on four tours and knows it all inside out. It's a massive mistake."
"The Lions is about getting the best of quality out of the players of these islands, not having an intransigent game plan that is low on subtlety but simplistic from the start.
"I just feel it has been incredibly hard watching because we are not seeing that spark we are used to seeing from the Lions because it is trying to play a game-plan that I do not know suits a lot of the players.
"It suits the Welsh players, and that is why there are 10 of them playing. It is really, really frustrating."
British and Irish Lions: Leigh Halfpenny (Wales); Tommy Bowe (Ireland), Jonathan Davies (Wales), Jamie Roberts (Wales), George North (Wales); Jonathan Sexton (Ireland), Mike Phillips (Wales); Alex Corbisiero (England), Richard Hibbard (Wales), Adam Jones (Wales), Alun Wyn Jones (captain, Wales), Geoff Parling (England), Dan Lydiate (Wales), Sean O'Brien (Ireland), Toby Faletau (Wales).
Replacements: Tom Youngs (England), Mako Vunipola (England), Dan Cole (England), Richie Gray (Scotland), Justin Tipuric (Wales), Conor Murray (Ireland), Owen Farrell (England), Manu Tuilagi (England).
Former Lions hooker Keith Wood says the decision to drop centre Brian O'Driscoll for Saturday's deciding Test with Australia is a "terrible mistake".