Timetastic comes up more than any other tool when people ask me how Who's OOO compares. It's a good product. Its pricing page says more than 275,000 people book their time off with it, the interface is clean, and at $1.50 per user per month it's cheap for SaaS. I built Who's OOO, so I'm not neutral here. What I can do is be specific, and link my sources for both tools so you can check them yourself.
The short version: Who's OOO costs nothing and runs on your own server. Timetastic needs no setup and has more features. Which one fits depends mostly on whether someone on your team is happy to run a small PHP application.
Feature comparison
| Who's OOO | Timetastic | |
|---|---|---|
| Price | Free, you pay for the server | $1.50/user/month (Business), $2.50 (Pro) |
| Free option | Yes, self-hosted | No, 30-day trial |
| License | AGPL-3.0 | Proprietary |
| Hosting | Your server or fortrabbit | Microsoft Azure, UK regions |
| Slack notifications | Yes | Yes |
| Approve or reject from Slack | Yes | Yes |
| Slack status sync | Yes | Not in their Slack docs |
| Microsoft Teams | No | Pro |
| SSO | No | Pro (SAML) |
| Mobile apps | No, web only | iOS and Android |
| Leave accruals | No | Pro |
| API | No, iCal feeds only | Yes, plus webhooks |
| Burnout alerts | No | Pro |
Plan details are from Timetastic's pricing page as of October 2026. Their Help Centre lists the same plans in pounds, £1.20 for Business and £2.00 for Pro, and explains the differences between the plans in this article. There's also a third plan, Rota & Time Clock, for shift scheduling, which I'm leaving out because Who's OOO doesn't do rotas at all.
What it costs over time
Timetastic charges per user, every month. There's a 30-day free trial and no free plan, and billing is month to month, paid monthly. Who's OOO costs nothing to use. You pay for the server, and that cost stays the same whether you have five people or fifty.
| Scenario | Timetastic Business | Timetastic Pro | Who's OOO on fortrabbit |
|---|---|---|---|
| 15 people, 1 year | $270 | $450 | €60 |
| 30 people, 1 year | $540 | $900 | €60 |
| 30 people, 3 years | $1,620 | $2,700 | €180 |
The fortrabbit figure is what my own instance costs: €5 a month for the smallest PHP, MySQL, traffic, backup and job components plus 1 GiB of storage. The line items are on fortrabbit's pricing details page. A small VPS costs about the same, for example DigitalOcean's $6 droplet. If you already have a server, adding Who's OOO costs nothing extra. Timetastic's prices are in US dollars and fortrabbit's in euros, so keep the exchange rate in mind when you compare.
The money isn't the whole picture, though. You're trading a monthly bill for an afternoon of setup and the ongoing job of keeping the app updated. For a five-person team, Timetastic Business is $7.50 a month, and a developer's afternoon may cost more than a year of that. But Timetastic's bill grows with every person you add, and the server bill doesn't.
Slack is where they differ most
Both tools work with Slack, but not in the same way. Timetastic posts a daily summary of who's off to a channel you choose, sends leave requests and notifications by direct message, and has a slash command for checking who's away. According to its Slack page, a manager can approve or decline a request with one tap from inside Slack.
Who's OOO posts new requests to a shared approval channel with Approve and Reject buttons, sends the employee a direct message when a request is approved or rejected from Slack (decisions made in the web app go out by email), and posts a weekly digest of who's out (Slack integration docs). So approving from Slack works in both.
The difference is the Slack status. While someone is on approved leave, Who's OOO sets their status to the leave type's emoji and text like "Vacation until Apr 18", and Slack clears it at the end of the last day of leave (how status sync works). A colleague who goes to message them sees it before they type anything. Nobody has to open a leave tool. I couldn't find anything similar in Timetastic's Slack documentation.
Status sync has two limits. It needs a paid Slack workspace, because free plans don't let an app change another user's status. And the sync job runs every 20 minutes on server time, UTC by default, so the status changes shortly after midnight on the server, not at midnight where each person lives. For someone in New York, that means their status shows them away from the evening before their leave starts.
Where Timetastic is the better choice
Timetastic has apps for iPhone and Android, and the pricing page includes them on every plan. Who's OOO is a web app. It works in a phone browser, but there's no native app and no push notifications. The interface is also English only for now.
Timetastic Pro adds a Teams integration with approvals, and SAML single sign-on with Azure AD, Okta, Auth0, OneLogin and Google Workspace (Pro features). Who's OOO has neither. Login is email and password, with optional two-factor authentication. If your company works in Teams or requires SSO, that decides it.
Timetastic Pro also supports monthly accrual. Who's OOO gives each person an annual allowance and adds it to their balance on a reset date. Unused days carry over in full, with no cap and no expiry (how balances work). If your policy needs monthly accrual or a carry-over limit, Who's OOO can't model it today.
For integrations, Timetastic has a REST API for admins, plus webhooks. Who's OOO has no public API. It has per-user iCal feeds you can subscribe to in Google Calendar, Apple Calendar or Outlook, and that's it for now.
Pro also has a Burnout Board that flags people who haven't taken much time off. Who's OOO has nothing like it.
And Timetastic needs no setup. You sign up and invite your team. Who's OOO needs someone to deploy it, set the environment variables, keep two background workers running, and create the first admin account by hand with an SQL insert. That's the real cost of self-hosting, and I'd rather say it upfront.
Where Who's OOO is the better choice
The price is the obvious difference. Who's OOO is free under the AGPL-3.0 license, there's no per-user fee, and nobody can move a feature you rely on to a more expensive plan.
The data stays with you. Absence records include sick leave, which is health-related data, and with Who's OOO it sits in a database you control. Timetastic hosts its service on Microsoft Azure in the UK South and UK West regions. That's fine for many teams, but it's still another company holding those records. Self-hosting doesn't remove the GDPR paperwork completely, though. Your hosting provider and your email provider are still processors under Article 28, and so is Slack if you connect it (Slack's DPA). What you remove is the leave software vendor.
You can read and change the code. It's Symfony 7.4 on PHP 8.5, and the full source is on GitHub. If something's missing, you can add it or open an issue. You also don't depend on a vendor's future. When a SaaS company changes its prices, gets acquired or shuts a product down, every customer has to deal with it. I wrote about that in a separate post on HR SaaS vendor risk.
Beyond Slack, the everyday features are covered: public holidays imported for more than 200 countries through Nager.Date, optional auto-approval after a delay you choose, minimum notice and maximum length limits, an org chart, and roles for admins, managers and employees.
Moving from Timetastic
There's no importer. You add each person in the admin panel, set their allowance and current balance, and they get an invitation email to set their password (user invitations). For a few dozen people that's manageable by hand. For a few hundred it would be slow, and that's a fair reason to stay on Timetastic for now.
Which one to pick
If you need mobile apps, Teams, SSO, accruals or an API, Timetastic is the better choice today, and I wouldn't try to talk you out of it. If your team does most of its talking in Slack, has someone comfortable running a PHP app on Docker or fortrabbit, and would rather pay a small fixed server bill than a fee per person, Who's OOO is worth an afternoon.
If you're still deciding whether to self-host anything at all, I covered that in self-hosted vs SaaS leave management. If you're moving off a spreadsheet, this post is closer to your situation. And if you want another open source option, I compared Who's OOO with TimeOff.Management.
In closing
Timetastic is a polished product with a lot of features, and for many teams the monthly fee is the right trade. Who's OOO is for teams that would rather own the tool and the data, and that care about seeing absences directly in Slack. It's open source, the installation guide walks you through setup, and the code is on GitHub. If it's useful to you, a star on GitHub helps keep the project going.