OEM advertising has become the go-to engine for mobile user acquisition in 2025, letting growth teams choose Cost Cap bidding to hold CPI and CPA in check or pivot to Event Cap bidding driven by Day-1 retention goals. With Xiaomi Mi Ads and Transsion now layering oCPC algorithms on top of Google’s Privacy Sandbox safeguards, marketers gain a future-proof way to scale profit without sacrificing privacy.
Two Flavours of Smart Bidding
| Lever | What You Tell the Algorithm | Typical KPI | Where It Lives |
| Cost Cap | “Bring me volume, but keep my average cost-per-install or cost-per-action at ≤ X.” | CPI / CPA | Xiaomi Mi Ads → CPA (billed by activations) setting keeps the rolling average under your ceiling. |
| Event Cap | “Optimise for a post-install goal; I’ll pay up to Y for each event or above Z% success-rate.” | Day-1 retention, purchase, registration | Xiaomi Mi Ads Post-link Metric Optimisation lets you set a Day-1 retention or purchase target and adjusts bids automatically. |
Quick analogy for beginners: Cost Cap is a thermostat — it stabilises average cost. Event Cap is an autopilot — it hunts for users who hit the deeper goal, even if the install price swings.
How the Mechanics Differ
- Bid calculation
- Cost Cap: the system floats bids up or down so the average CPA/CPI stays under the target.
- Event Cap: the system predicts the likelihood of the chosen in-app event (e.g., Day-1 retention) and bids higher where probability × payout ≥ target, using post-back data from MMPs like AppsFlyer or Adjust.
- Cost Cap: the system floats bids up or down so the average CPA/CPI stays under the target.
- Learning requirements
- Cost Cap campaigns start spending almost immediately — handy when you launch in a new GEO with little history.
- Event Cap needs a stream of in-app events (Xiaomi recommends ≥ 50 target events per ad set per week) to keep the model stable.
- Cost Cap campaigns start spending almost immediately — handy when you launch in a new GEO with little history.
- Budget sensitivity
- Set Cost Cap too low and delivery stalls; set Event Cap too aggressive (e.g., +5 % jumps each edit) and you reset the learning phase.
- Set Cost Cap too low and delivery stalls; set Event Cap too aggressive (e.g., +5 % jumps each edit) and you reset the learning phase.
Platform-by-Platform Status (Q3 2025)
| OEM network | Cost Cap availability | Event / Deep-event bidding |
| Xiaomi Mi Ads | Live via CPA/CPI targets in the console. | Live: Day-1 retention / purchase optimisation under Post-link Metric Optimisation. |
| Transsion Eagllwin | CPM & CPC are public; oCPC (optimised bidding) is whitelist-only as of Aug 2022. | Event-level optimisation is still in closed beta — plan tests with your AM. |
| OPPO ColorOS Ads | Roll-out in progress; mirrors Meta-style Target CPA. (No official docs yet — check with your rep.) | Not publicly documented; expect parity with Xiaomi later in 2025. |
Which Lever Should You Pull?
| Your immediate goal | Recommended lever | Why |
| Launch in a brand-new region with tight CPI targets | Cost Cap | Fast delivery, predictable averages. |
| Scale ROAS or hit pay-back windows | Event Cap | Algorithm seeks high-quality users who convert or stick. |
| Mature title needs both reach & depth | Start Cost Cap for data → migrate to Event Cap once ≥ 50 tracked events / week. |
Four-Step Checklist to Get Started
- Instrument events – pass retention, purchase or level-complete events from your MMP to the OEM console.
- Benchmark honestly – set the cap slightly above your blended historical CPI or cost-per-event; too low throttles delivery.
- Feed the model – aim for at least 50 qualifying events per ad set per week.
- Iterate weekly – if volume stalls, widen device tiers or relax the cap by 10-15 %. Small 5 % tweaks keep Xiaomi’s Event Cap model in learning without a hard reset.
Bottom Line
OEM traffic is no longer a bargain-bin install source. Cost Cap guards your spend like a thermostat; Event Cap hunts for real value. Master both — starting with Xiaomi’s mature tool-set — and you’ll convert on-device impressions into a dependable profit engine for your 2025 growth plan, all while staying ahead of the Privacy Sandbox shift away from legacy IDs.
