ox is good for strength gains, but it will also bring mass (not that much, but you say you don't want weight increase)...although it's side effects are mild and can be easily countered (liver...r-ala,NAC, milk thistle)
now, a known beneficiary for strength for powerlifters is halotestin and cheque drops...
dosage: 20-30mg for around 4 weeks before competition/meet
these give mad strength with almost no weight increase...but they're 17-aa hence take care of your liver...and i believe fluoxy- group by itself is rather toxic..