If you need Outlook Calendar API alternatives for scheduling integration, the right choice depends on whether you want:

• Calendar sync
• Availability lookup
• Booking / scheduling links
• Bi-directional event management
• A multi-calendar abstraction

Here are the main options.

1) Microsoft Graph Calendar API

This is the official modern replacement for older Outlook REST APIs.

Best for: native Microsoft 365 / Outlook integration
Features:

• Read/write events
• Free/busy lookup
• Webhooks / change notifications
• Recurring events, attendees, time zones
• Works with Outlook.com and Microsoft 365

Pros:

• Official and supported
• Full calendar control
• Strong enterprise support

Cons:

• Requires Azure app registration and OAuth
• Microsoft-specific

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2) Google Calendar API

If your scheduling needs include Google users or cross-provider support, this is the main alternative.

Best for: Google Calendar integration and general scheduling apps
Features:

• Event CRUD
• Free/busy
• Watch notifications
• Recurring events, conference data, guests

Pros:

• Mature API
• Good ecosystem
• Common in scheduling products

Cons:

• Google-specific OAuth and permissions
• Not a direct Outlook substitute

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3) CalDAV

A standards-based calendar protocol supported by several providers.

Best for: interoperability and custom calendar clients
Features:

• Read/write calendar events
• Sync support through standard protocol

Pros:

• Open standard
• Not tied to one vendor
• Useful for multi-provider compatibility

Cons:

• Provider support varies
• Can be harder to implement than Graph/Google APIs
• Free/busy and notification support may be inconsistent

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4) Scheduling platforms with APIs

If your goal is booking meetings rather than raw calendar operations, consider scheduling products.

Examples:

• Calendly API
• Cronofy
• Nylas
• Chili Piper
• OnceHub

Best for: scheduling workflows, booking links, availability management

Pros:

• Faster to integrate
• Handles calendar connections, availability, and invites
• Some support multiple calendar providers

Cons:

• External dependency
• Cost
• Less control than direct calendar APIs

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5) Nylas Calendar API

Nylas provides a unified API over multiple calendar providers.

Best for: apps needing one API for Outlook + Google + others
Features:

• Unified event management
• Availability
• Webhooks
• Email/calendar integrations

Pros:

• Abstracts provider differences
• Speeds up integration

Cons:

• Paid service
• Vendor lock-in risk
• You depend on Nylas as middleware

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6) Cronofy

A strong scheduling API focused on calendar connectivity and availability.

Best for: scheduling and booking apps
Features:

• Cross-calendar availability
• Event management
• Booking workflows
• Multiple provider support

Pros:

• Good for scheduling use cases
• Unified model across providers

Cons:

• Paid
• Not as flexible as direct provider APIs

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7) ICS / iCalendar file-based workflows

If you only need lightweight calendar exchange, you can use .ics files and calendar invitations.

Best for: simple event sharing or invite generation
Pros:

• Very simple
• Widely supported

Cons:

• Not suitable for real-time sync or availability
• Limited for complex scheduling integration

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Recommendation by use case

If you need direct Outlook integration:

• Microsoft Graph Calendar API

If you need cross-provider support:

• Nylas or Cronofy

If you need a standards-based open option:

• CalDAV

If you mainly need booking workflows:

• Calendly API, Cronofy, or Nylas

If you need Google + Outlook support and don’t want to build provider logic:

• Nylas or Cronofy

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Important note

The old Outlook REST API is deprecated; for Microsoft calendars, use Microsoft Graph instead.

If you want, I can also provide:

1. a comparison table of these options,
2. a recommended architecture for your app, or
3. a step-by-step integration plan for Outlook scheduling.