OpenMPI is built on Magi with SLURM support. To use OpenMPI, you must load the correspondig module. Feel free to browse the /softs/modules/compiled/gcc/<gcc_version>/openmpi/ to check available versions of OpenMPI. For example :

 nicolas.greneche@magi3:~$ module load /softs/modules/compiled/gcc/10.2.1/openmpi/4.1.2

Then, you can compile your OpenMPI codes :

 nicolas.greneche@magi3:~$ mpicc -o hellompi hellompi.c

The last step is to write a submission script for slurm (here run.slurm) :

 #!/bin/bash
 #SBATCH --job-name=hello
 #SBATCH --output=slurm.out
 #SBATCH --error=slurm.err
 #SBATCH --mail-type=end
 #SBATCH --mail-user=nicolas.greneche@univ-paris13.fr
 #SBATCH --nodes=2
 #SBATCH --ntasks-per-node=2
 
 srun ./hellompi

You can refer to this documentation for more informations on job submission.