Well, by the time I got the first reply above I was already in Leconi (so out of email range) awaiting a ride. Thanks anyway to one and all. I thought I updated this thread last year but it appears to have slipped my mind. Not sure how good this information still is, but here goes (cut and pasted from a couple of my posts on another site):
I crossed the border going east from Leconi to Kabala (Gabon) to Lekety (Congo) to Oyo. I had arrived in Leconi on a Friday, and I gather transport is found more easily during the week (and arguably found more easily in Franceville than in Leconi). I had to wait a few days. Do your border formalities at Leconi (I had a Congo visa already so can't help if you don't have it in-hand).
Though transport is by no means regular at the moment, a paved highway is being built now between Bongoville and Oyo. It may be no coincidence that Bongoville is the hometown of Gabon's President-for-life Omar Bongo, while Oyo is just next to the hometown of Congo President Denis Sassou-Nguesso--Bongo's father-in-law. Isn't that sweet? Anyway, the 300km or so from Leconi to Oyo can now be done in a long day's drive (about 12 hours in my case) during the dry season, with the hitch being finding a ride if you don't have your own transport. Your best bet is to get a ride with a merchant, or with one of the highway company's trucks, or with one of the infrequent cars that take passengers.
The best and fastest route to Brazzaville from eastern Gabon is currently the one I took (from Oyo it's a few hours on tarred road to Brazzaville), and it will only get faster if the road is finished. However, if you want to just take the first transport that comes along then the other two options from Leconi are: (a) to go farther south to Lekana (Congo), which most people said involves a ferry crossing, and get a bus from there to Ngo and then Brazzaville (all night drive on bad road to Lekana, then long busride to Brazzaville); or (b) to go up north to Ewo and then on to Oyo from there, but this could be a five-day thing.
There is still the most-used route closer to the coast, but this interior route is a good alternative and seems likely to become a major crossing point once the road is finished.
So eventually a couple of nice guys from Chad took me across at Lekety. It cost 50,000 CFA, and by that time I wasn't interested in arguing about the price. There was also an added cost in the form of really hefty "extra fees"--I had never paid these before in my life, preferring to wait officials out rather than encourage this behavior, but in this situation I was with a couple of foreign guys trying to get their stuff to market (and doing me a favor). I didn't want to inconvenience them, so I ponied up 5,000 CFA at Kabala (the Gabon side of the border), and 5,000 at Okoyo in Congo. Apparently everyone has to pay this whopping amount, so this "tax" may be the reason why the road is less used than it might otherwise be--one hopes this will change once the roadwork is finished. I have no idea whether stalling and argument would work as well here as they have for me elsewhere in Africa, but as I say I didn't try.
We were in a good 4WD vehicle and it had not rained much recently. The 300km trip took about 12 hours, and we were not in a rush. The road is mainly sand, not dirt, so a little rain is not as bad a thing as it could be.
We left Leconi at about 06:45, arriving at Kabala around 07:30. Reached Mbie at around 08:30, where we stopped for awhile. We passed by Amina and then Ongali by about 10:00, and after another stop were at Lekety (maybe 70km on from Leconi) by about 10:45 for customs and police. Okoyo was less than an hour further on, but between formalities at Lekety and at Okoyo we ate up another hour and a half or so. By 13:00 we headed northeast, arriving at Boundji about 16:20. Boundji is abut 80km from Oyo, and about half of the remaining distance would be on good tar road. By 18:00 we were at Obouya and the blacktop road, and at Oyo by 19:00. The route is pretty, and I have never seen such numbers and varieties of butterflies as on this route--huge clouds of them would lift off the road on our approach. We also got to sample some damned good papayas along the way.
At Oyo I stayed at Alima Palace, maybe 300 meters south of the main drag (there is a prominent sign for the place to the right as you come from the direction of Edou, the presidential village). It cost 10,000 CFA for a small room with ensuite (dank and leaky) bathroom and a/c. The place was fine for a night, but I suspect it sees more use from clients who are intent on something beyond sleeping. If you go out at night remember to take your torch for the long walk to the main road.
There is also a restaurant and hotel right on the main drag (on the left side, opposite the sign for Alima Palace and next to the Celtel office), with decent, plush rooms clearly designed for adult entertainment. The name is "Chez" something or other. The food is good--prices run maybe 1,500 CFA or so for a meat-plus-sauce-plus-bread meal and a
There is as yet no internet connection in Oyo. A Celtel sim card will set you back 2,000 CFA. I bought mine in Oyo. For some reason mine worked well with my Nokia phone but not with the Motorola Razr even though the latter is supposedly more versatile. I knew there was a reason to carry two phones...