O'Brien netted her third goal in four league games, while late heroics from Leitzig confirmed our second Barclays Women's Super League win of the season, climbing up to ninth in the standings heading into the break as a result.
There were two changes from the side that lost to Manchester City on Filbert Way last weekend, as Sari Kees and Emily van Egmond coming in for Celeste Boureille and the injured Hlín Eiríksdóttir.
An even first half
O'Brien came closest for the Foxes in the first half.
This year's newly-promoted side weren't to be an easy task this Sunday, having already won half of their first 10 games of the season. And they met a City side still on the hunt for a second win of the term.
But there was very little to separate the two sides during the first period, with London City edging the stats in terms of possession, but the Foxes looking to cause problems on the break.
The best chance of the first period came when O'Brien was set through on goal having curled her run to open up a shooting opportunity, but, from a tight angle, the 'keeper was able to get behind it and turn it away from danger.
O'Brien's scoring streak continues
A third goal in four league games.
Five minutes into the second half, O'Brien had the ball in the net when she chipped the goalkeeper from a long distance, but a glance to the line showed a raised flag, signalling offside.
The warning sign was there for London City and O'Brien did get herself on the scoresheet not long after the hour mark when a cross field ball was expertly nodded across goal by Asmita Ale, into the path of the forward, who took it around the 'keeper before tapping in from close range.
A third goal in four WSL games, O'Brien will hope her red hot form in front of goal continues after the winter break.
Leitzig's late heroics
As the board signalled eight additional minutes, City were set in to defend the win, but in the dying stages, Janice Cayman gave away an unfortunate penalty, giving Kosovare Asllani the chance to rescue a late point.
With the sun in her eyes, Leitzig pulled off a blinding save, diving to her left to get behind the spot-kick and keep Leicester in front.
After 100 minutes of football at King Power Stadium, the referee's whistle blew and marked a perfect end to 2025 for the Foxes.