USNR MOBs are likely to hold steady at 2200-ish for at least several more years. You can do your own math based on force structure, five-year dwells and who has what clearances.
As was mentioned a TS/SCI is going to be required for most (increasing with rank) officer MOB slots, so that narrows the odds if you have one. But if you don't have a TS/SCI it narrows your options for O5 pay billets, and makes it significantly harder to get an O6 pay billet...pick your poison.
Your reserve unit is not a consideration at all for individual MOBs unless it's a priority hardware unit, or scheduled for a unit-wide MOB. It's possible for an NR augment unit to get decimated by random MOBs.
O6 MOBs appear to be largely filled with volunteers right now (many of them seem to lose their civilian jobs around age 50 after playing hard in the NR for years...).
Regarding active duty IA's...those are doled out from on high, and then the unit can typically fill them as it sees fit. There was an issue about ten years ago with AC units hiring reserves on a VOL basis (ADSW or MOB) ostensibly for a local job and then immediately assigning them to fill an AC IA in the 'stans. Reserve force & PERS put the kabosh on that practice...if they advertise a certain billet and location, that should be the deal. If a billet falls out last minute, they should give you the choice of another billet if available, or just go home.