Hall had scored winners against Oxford twice already this season and he set the Shrimpers on their way when his 19th-minute free-kick hit the net for his 14th goal of the campaign.
Mohsni then got the crucial second with a well-placed header from Peter Gilbert's cross, ensuring Southend could yet claim third place in npower League Two on the final day of the season this weekend.
Boss Sturrock said: "Ryan's a quality player. I've been pleased with him, with his work-rate as well.
"Mohsni's unbelievable in the air. He's the best centre-half I've ever signed."
He added: "It's been a scary season, but it's fantastic that it's going to the last game of the season. Two teams have to lose and draw next week and we have to win. Momentum-wise, we're going in the right direction for the play-offs."