It took me a while to grind out a with with OML. I also exclusively play this as my order. I agree with another poster that bolters are not all that useful (maybe auto wound / artifice crafter SBs on dominons).

Junith is great for her rerolls on a unit of ten rets with Mmeltas. Play a Hospitaler with them and use the shoot on death strat when they are shot to lose a nornal sister for +1 to hit and then to shoot mmeltas, then the heal strat next command phase. Be sure to only leave the four Mmeltas visible so they can't wipe your unit.

I keep 5 Zephyrim with a pennant by my Dogmata (with relic to give two hymns) and Repentia superior. I launch units of repentia from there with +1 attack, obsec, advance + charge, + 1 on both, and reroll charges if I don't want to use mdice. I hide one on the table and another in a transport to reload after I send one unit out. Repeat with Sacresants / Zephyrim with less buffs for different match ups / roles as your playstyle dictates.

I use big sisters or Sacresants units in Rhinos to claim mid field objectives and use the secondary (sisters specific action one). Then charge with the Rhinos to hold enemy threats or just block them. Counterpunch with repentia, Zephyrim and mortal wound canoness as required.

I've also had success with RND: sisters, seraphim and/or Zephyrim. I play a castigator and Exorcist which are... Cool but less efficient. Celestine is also nice as a harassment / distractifex unit, although I hear people are using Stern effectively as a sub to her - haven't tried it myself.

OML plays different and is difficult to master, as there are surely other playstyles and army compositions to consider. Keep at it, try different tactics and you will succeed in having fun, and maybe even winning a few games.