You don't need mysql's extra features for an otserver, it mainly slows it down. For the tasks an otserver needs to do in a database, sqlite would probably better than mysql because it's sometimes twice as fast for common operations.
Sqlite is easier, faster, uses less memory, uses less cpu time and is good enough for even a big otserver.
These are wrzasq's benchmarks. The original tests were done with 100x fewer iterations for SQLite, so I moved the decimal point back twice on SQLite tests to compensate.mysql:
INSERT: 29.12858s
SELECT one result: 51.60862s
SELECT multiple results: 0.820717s
Advanced test: 41.86564s
sqlite:
INSERT: 1706.281s
SELECT one result: 007.51007s
SELECT multiple results: 000.83878s
Advanced test: 005.29901s