Five room house, four leases signed, and I cannot figure out how to price the fifth without undercutting the four I already have

The assumption doing the most work in your $580 framing is that tenants benchmark against each other's rates rather than against the room itself. That is partly true, but the comparison they actually make is price-per-what-I-get. If you can make the product distinction visible and defensible, the rate difference follows from the product, not from a discount you granted someone else.

The closet absence and the 118 square feet are real differentiators. The question is whether you have made them legible in the listing and in the physical space. A room without a closet listed at $600 reads as a bad deal on the same terms. A room without a closet that is explicitly framed and priced as a different tier, and has a wardrobe unit in it so the closet problem is solved, is a different product. That wardrobe costs you $150 to $300 and changes the conversation from "why is that room cheaper" to "that room comes furnished differently."

On the tenant who texted you: she does not have a case until something signs. What she has is anxiety about fairness, which you can address directly without discussing what the fifth room will rent for. You confirm her rate is her rate, the fifth room is a different configuration, and the pricing reflects that. You do not owe her the number in advance.

The risk you did not name is what happens if the fifth room sits through the lease renewal cycle for the four who are in. If any of them are on short terms and go to market while the room is still vacant, you lose the rate anchor on the whole property, not just the fifth room. How long are the existing four leases, and are they staggered or co-terminating?

The tenant who texted you has no lease clause that ties her rate to what others pay, so her leverage is exactly zero unless you give her some.

The framing problem is easier than you think. A room without a closet is not a cheaper version of the other rooms, it is a different product, and you can list it that way plainly: "no built-in storage, smaller footprint, priced accordingly." Anyone comparison-shopping on a tour already knows it is smaller because they can see it. The math a current tenant does in the hallway is not your problem to solve in advance, it is their problem to reconcile with their own signed lease. What does your lease say about rent adjustments or most-favored-tenant provisions, because if nothing is in there, the conversation that tenant started has no foundation.

The real number to work out is the breakeven on vacancy duration. At $675 average across four rooms your monthly gross is $2,700. One month empty at $550 still gets you $2,200 in month one and $2,750 every month after. One month empty at $600 gets you $2,400 in month one and $2,700 every month after. The difference between those two futures is $50 a month in perpetuity against a one-time cost of roughly half a month at the lower price, which pencils at somewhere around three months to recover. If your carry on the fifth room is light, waiting costs less than you probably feel like it does right now.

The 19 days with zero interest at $600 is the real signal. What does the listing say about storage, because that is the objection you need to answer before the showing, not on the doorstep?

What are the other four rooms pulling in comparable Owasso co-living houses right now?