Algeria went 2-0 up at half time through strikes from Sofiane Feghouli and Ishak Belfodil. The same goal scorers then added another goal each to make it 4-0 on the hour, before Mahrez was introduced as a substitute with 25 minutes left and Oman grabbed a consolation.
Elsewhere Liam Moore joined England’s Under-21s side in Middlesbrough as they warmed up for the summer's European Under-21s Championships with a tough test against potential title challengers Germany.
Moore, who played a key role in the 1-0 win over the Czech Republic last week, was an unused substitute on this occasion, but it didn’t’ stop Gareth Southgate’s men securing a morale-boosting victory over the Germans.
They twice went behind to goals from Philipp Hoffman, but strikes from former City loanee Jesse Lingard, Nathan Redmond and James Ward-Prowse handed them victory.
There was further international involvement in the early hours of Tuesday morning, as Wes Morgan took to the field for Jamaica as they overcame Cuba 3-0 in Montego Bay.
Daniel Gordon, Romeo Parkes and Darren Mattocks had completed the scoring as early as the 34th minute as the Reggae Boyz coasted to victory and made an encouraging start to preparations for the 2015 Copa America tournament.