db.setConnectOptions("CHARSET=WIN1251");

#define TRANSACTION(x) (QSqlDatabase::database().driver()->setProperty("Transaction",(x)))
#define TRANS_SELECT "TAM=amRead, TIL=ilReadCommitted, TLR=lrNoWait, TFF=0"
#define TRANS_UPDATE "TAM=amWrite, TIL=ilConcurrency, TLR=lrNoWait, TFF=0"
#define TRANS_REPORT "TAM=amRead, TIL=ilConcurrency, TLR=lrNoWait, TFF=0"
#define TRANS_DEFAULT "TAM=amWrite, TIL=ilConcurrency, TLR=lrWait, TFF=0"
// .............
TRANSACTION(TRANS_SELECT);
QSqlQuery query.prepare(".........

#include <QTextCodec>
class CTextCodecWIN1251 : public QTextCodec
{
private:
    QTextCodec *win1251Codec;

public:
    CTextCodecWIN1251();
    ~CTextCodecWIN1251();

    virtual QByteArray name() const;
    virtual QList<QByteArray> aliases() const;
    virtual int mibEnum() const;

protected:
    virtual QByteArray convertFromUnicode( const QChar * input, int number, ConverterState * state ) const;
    virtual QString convertToUnicode( const char * chars, int len, ConverterState * state ) const;
};
CTextCodecWIN1251::CTextCodecWIN1251()
{
    win1251Codec = QTextCodec::codecForName("Windows-1251");
}
CTextCodecWIN1251::~CTextCodecWIN1251()
{
}
QByteArray CTextCodecWIN1251::convertFromUnicode(const QChar * input, int number, ConverterState * state) const
{
    if(!win1251Codec)
        return QByteArray();
    return win1251Codec->fromUnicode(input, number, state);
}
QString CTextCodecWIN1251::convertToUnicode(const char * chars, int len, ConverterState * state) const
{
    if(!win1251Codec)
        return QString();
    return win1251Codec->toUnicode(chars, len, state);
}
QByteArray CTextCodecWIN1251::name() const
{
    return QByteArray("WIN1251");
}
QList<QByteArray> CTextCodecWIN1251::aliases() const
{
    QList<QByteArray> list;
    list << QByteArray("WIN-1251") // Эти алиасы не обязательны
         << QByteArray("CP1251")
         << QByteArray("CP-1251")
         << QByteArray("1251");
    return list;
}
int CTextCodecWIN1251::mibEnum() const
{
    if(!win1251Codec)
        return 0;
    return 2251;
}


...
    CTextCodecWIN1251 * p = new CTextCodecWIN1251();
...
    db.setConnectOptions("ISC_DPB_LC_CTYPE=WIN1251");