I don't have OEGB, but I believe they can be sexed with traditional methods... Boys grow wattles and get red combs before girls, at 15 week boys get pointed feathers girls stay rounded.
Silkies are a bit trickier. On my pet quality silkies I can look at the crest placement. The younger they are the easier it is. Boys crests are farther back I have heard this doesn't work for all lines. It might take 15 weeks or so... Crowing may or may not be the first sign of a male. Combs get a bit raised and darken. Crests get streamers, feathers that are taller than other in the crest feathers. Tails get fuller, birds get larger and stand more up right.