I went with Boros. BUT...
My intuition when I first saw Osgir was correct; Osgir, the Reconstructor is no joke and creates one of the most unique forms of Card Advantage I've ever experienced.
So, if you like drawing cards, smashing face, and creating incredibly potent value engines with potential COMBO synergies in the two worst objective colors in MTG, check out what Osigr does.
Since Boros is light on tutors, we supplement this weakness with raw card draw, and I need to stress to you the importance of the Eggs that are in the deck. "Eggs" in MTG refers to cheap Artifacts that either have a self-sacrificing ability that draws you cards OR draw cards when they ETB. This list of cards includes: Mishra's Bauble, Conjurer's Bauble, Ichor Wellspring, Arcum's Astrolabe, and Golden Egg. These cards are absolutely wonderful with Osgir out in play and the best part is that they completely fly under the radar of your opponents. More often than not, you'll cast one of these Eggs, sac it to Osgir's ability, then use his second ability to make two token copies of that card. Essentially, you're able to draw 3 cards for 3-4 mana most times, and this is one of the primary engines in the deck for card draw.
In regards to the specific infinite turns combo in the deck, the lovely folks on the Boros Discord server have figured this one out and I'll be putting it in spoilers for ease of understanding:
1. Activate Osgir's ability for X=5, exile Ugin's Nexus: 2 Nexus tokens; 1 dies to legend rule [1 extra turn][0 Mirrors]
2. Pop 2nd Nexus with Osgir [2 extra turns][0 Mirrors]. The OG Nexus will go to exile.
3. Move to next turn [1 extra turn][0 Mirrors]
4. Activate Osgir's ability for X=5, exile Mirror of Fate: 2 Mirror tokens [1 extra turn][2 Mirrors]
5. Crack a Mirror token copy, put nothing back [1 extra turn][1 Mirror] and exiling your entire library
6. Crack the last Mirror token copy, putting back Codex Shredder, Nexus, and Mirror (which you exiled via Osgir's ability in step 4) [1 extra turn][0 Mirrors]. Remember that you can stack your library in any order you want via Mirror of Fate so that you always draw the cards in whatever order you need.
7. Move to next turn [0 extra turns][0 Mirrors]
8. Draw Shredder, play Shredder [0 extra turns][0 Mirrors]
9. Use Shredder to mill Nexus [0 extra turns][0 Mirrors]
10. Activate Osgir's ability for X=5, exiling Nexus: 2 Nexus tokens; 1 dies to legend rule [1 extra turn][0 Mirrors]
11. Pop 2nd Nexus with Osgir [2 extra turns][0 Mirrors]
12. Move to next turn [1 extra turn][0 Mirrors]
13. Use Shredder to mill Mirror [1 extra turn][0 Mirrors]
14. X=5, exile Mirror: 2 Mirror tokens [1 extra turn][2 Mirrors]
15. Crack 1 Mirror, put back SSG, Nexus, Tormod's Crypt, and Mirror [1 extra turn][1 Mirror]
16. Move to next turn [0 extra turns][1 Mirror]
17. User Shredder to mill Nexus [0 extra turns][1 Mirror]
18. Activate Osgir's ability for X=5, exile Nexus: 2 Nexus tokens; 1 dies to legend rule [1 extra turn][1 Mirror]
19. Pop 2nd Nexus with Osgir's first ability [2 extra turns][1 Mirror]
20. Move to next turn [1 extra turn][1 Mirror]
21. Draw Tormod's Crypt, play Tormod's Crypt [1 extra turn][1 Mirror]
22. Use Shredder to mill Mirror [1 extra turn][1 Mirror]
23. X=5, exile Mirror: 2 Mirror tokens [1 extra turn][3 Mirrors]
24. Crack Crypt, targeting yourself (exiles crypt during resolution) [1 extra turn][3 Mirrors]
25. Crack 1 Mirror, Nexus, Tormod's Crypt, and Mirror [1 extra turn][2 Mirrors]
26. Return to Step 17
If that's not too convoluted for you (I'm still wrapping my head around it even as I type this up in my insomniac induced state), the net result is taking infinite turns. You can also pretty much cast any spell from your deck an infinite number of times as well (if I'm reading this right) by including Daretti, Scrap Savant in one of the piles, but I still need to wrap my brain around this.
There's an alternate semi-infinite combo line that requires WAY more cards but is WAY more straightforward is Anointed Procession/Rings of Brighthearth/Battlemage's Bracers + Karn, the Great Creator + Ugin's Nexus + Osgir, the Reconstructor + Daretti, Scrap Savant/Key to the City. Here's how it goes:
- You cast Nexus, sac it to Osgir's first ability, take an extra turn as per Nexus' triggered ability. The Nexus goes to your exile zone.
- Activate Karn's second ability to return Nexus from your exile zone to your hand.
- Activate Daretti's ability to discard the Nexus to your 'yard.
- Activate Osgir's second ability for X=5, targeting the Nexus in your 'yard.
- Copy Osgir's ability with either Rings of Brighthearth or get 4 Nexus tokens via Anointed Procession.
- 3 of them will die to the Legend rule, stacking up 3 extra turns for you.
- Sac the last Nexus token to Osgir's first ability, for a total of 4 stacked Extra Turns. You must have mana to do this otherwise you won't be able to take your stacked turns due to the Nexus' first ability.
- Proceed to take the first of your 4 extra turns. This will give you plenty of time to use Karn's first ability to restore his loyalty and activate this loop again during your fourth extra turn.
- You can protect yourself from decking out with Mirror of Fate and Tormod's Crypt (the Crypt will exile your 'yard and the Mirror will allow you to create a perfect 7 card deck of your favorite cards).
This deck is VERY subtle and I feel like I'm still discovering interactions. Not surprising, given the fact that Osgir, the Reconstructor was only available in paper as of yesterday...