For polish, user-friendliness, and staying consistent commonly used phrases that reflect the desired functionality, maybe consider replacing "Top Up OST" with any of the following...
Alternatives to "Top Up OST"...
- "Add OST"
- "Add More OST"
- "Request OST"
- "Increase OST"
- "Increase OST Balance"
Again, not a deal breaker by any means. Every little step we can take to make the experience slightly more intuitive can go a long way. Keep up the great work team. Proud to be a part of something special like this.