Outside half O'Gara said the wind made hitting the winner easier than the one he scored against Northampton last weekend following his side's 27-24 Heineken Cup win.
O'Gara nailed his second winning drop goal in as many weeks after a end-to-end Pool One battle played in Toulouse.
First half tries from hooker Brice Mach and replacement winger Pierre-Gilles Lakafia gave Castres an 18-10 half-time advantage, before Peter O'Mahony and Will Chambers added to Doug Howlett's earlier try for Munster.
Castres levelled things at 24-24 only for O'Gara to fire over from 40 metres out with the last kick of the game, meaning Munster and the Scarlets lead the way at the top of the pool.
"I think it was a lot easier this week to be honest," said O'Gara after his 99th match in the Heineken Cup.
"I had a big wind behind me. I nailed the kick last week. This one today was a case of letting the wind do the work.
"I just had to get the ball inside the right hand post and the wind would look after it.
"Last week if I had 50 balls I wouldn't kick as sweet a one as I hit. So today was just picking a spot inside the right hand post and just trusting your technique."
However, the Ireland star felt his side should not have had to rely on his last-minute heroics after giving Castres an 11-point head start.
"I did enjoy it. It was nicely struck but I think we probably shouldn't have put ourselves in that position," he added.
"I think we were good at times, poor at times today so we have plenty of areas to look at. But the most important thing is it's two wins out of two.
"This is a huge competition for us and maybe a year ago I think we would have crumbled in that situation."
Munster head coach Tony McGahan was full of praise for O'Gara, after watching his number 10 condemn Castres to the same fate as Northampton a week ago.
"We talked about 41 phases last week to get the drop goal and to do it two weeks on the bounce is certainly incredible," said the Australian.
"We relied on the other 22 players to make sure they were in a position to put Ronan in that position and, as I said last week, he's certainly a special player. To do it two weeks in a row is an absolutely outstanding effort."
Munster failed to win away from home in last season's Heineken Cup but McGahan feels his team took some lessons on board after failing to qualify for the quarter-finals for the first time in 13 years.
"I think it's a little bit of belief and probably a little bit more confidence," he said of his side's improvement.
"We went through some harsh lessons last year as a group and so you certainly want to be learning those in going forward."
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