Okay, some random tips. I don't know what in particular you're struggling with so, yeah:
I personally find that once you're past their defence (a stiff arm especially) the tsurite grip is the most important in executing Osoto Gari. I like to create kuzushi by driving my knuckles hard up under the jaw as I enter and directing uke's head up and diagonally backwards (same idea as the Tenri wrist) - I've actually thrown just with this in shiai with my opponent having avoided the reap but been off-balanced and continued to the ground as I pushed their head down to the mat. Most people today seem to throw with a HCG, but I find a standard lapel grip to be better personally (unless you're much taller than your opponent) as it gives you more options and isn't telling your opponent that you're probably going to attempt Osoto Gari.Bump your chest hard into their's to further aid the kuzushi and to ensure that you're close enough. With the hikite I pull down and around leading my opponent to where they will land.
Execute the reap at a 45degree angle, not straight ahead like you're always taught as a beginner.
Reap with a pendulum action through your whole body, don't just peck at the leg. Point your toes.
Osoto Gari requires full commitment, or you'll find yourself getting countered (often by Osoto Gari). Don't be afraid to blast it and smash your opponent hard to the mat. Also don't be afraid to land on your opponent, even if you're a big guy (I weigh >100kg myself). Don't be afraid to turn your Gari into a Gake and drive, but this should be a secondary option if the reap is well resisted.
The throw really requires a decent set-up as it can be easy to see coming. I like Ouchi Gari to get them moving backwards, then try to take their leg as they're moving it backwards but before they plant it on the ground (easier said than done, but that's the ideal). Ko Uchi Gari is good too because most people will try to step back off of it thus setting up Osoto Gari.
Work on your Hiza Guruma and Sasae Tsurikomi Ashi. Often, especially if your opponent knows you like Osoto Gari, they will push into you as you enter for Osoto Gari - this is a golden opportunity for Hiza or Sasae. Harai Goshi is nice too if you have started your reap when they push back into you, again as they resist by pushing against you Harai Goshi takes them with their own momentum.