MySQL's full-text search capability has few user-tunable parameters.
For Osclass users, item title and item description have a fulltext index, that do search more effective and speed up.
The minimum and maximum lengths of words to be indexed by default is 4 (4 characters), that means, only words more or equal than 4 will be indexed.
If you like to change the minimum and maximum lengths of words to be indexed, you need update your mysql config file.
After this, if you modify full-text variables that affect indexing (ft_min_word_len, ft_max_word_len, or ft_stopword_file), you must rebuild your FULL-TEXT indexes after making the changes and restarting the server. To rebuild the indexes in this case, it is sufficient to do a QUICK repair operation:
REPAIR TABLE tbl_name QUICK;
Source: Fine-Tuning MySQL Full-Text Search