The company posted a post-tax loss of £277m as of 30 September, against a loss of £387m last year.
Earnings before interest, tax, depreciation and amortisation grew to £378m, compared with £135m the year before. Total revenue grew by 11.5% in the six-month period to £2.34bn, from £2.09bn in the same period last year.
The operator attributed the revenue increase to growth in its customer base in both the UK and Germany.
MmO2's total customer base grew by 8.9% in the period to 18.3 million, while revenue from services were up by 16.3% to £2.09bn. Data revenue accounted for 15.6% of service revenue in the second quarter and 14.6% in the first quarter.
The company predicted that by the end of 2004, 25% of group revenue would come from data services.
Net debt at the group fell to £609m at the end of September, compared with £617m at the end of March.
The mobile operator, which owns networks in the UK, Germany, the Netherlands and Ireland, plans to increase its investment in its third-generation (3G) network this year and is aiming to launch 3G services in the second half of next year.
The company predicted continued "strong performance" for the full year with further growth in the German market in particular. It noted, however, that the UK mobile market remained "challenging" and warned that future revenue could be affected by an impending ruling from the Competition Commission, which is looking at the cost of calls made between rival network operators.