As others have noted, there is no comprehensive guide to all O gauge manufacturers/production over the past couple of decades, nor is there likely to be one in the near (and perhaps distant) future. It would be a major and very time consuming--not to mention costly--undertaking for any individual or firm. Even if it was strictly an online publication, those same limitations would still hold true. In large part, that is because some manufacturers, in particular, have been very productive since around 1990 or so, and an accurate presentation of what has been made (and items that were supposed to be made, but never actually were) would encompass a huge volume.
My advice, if O gauge is your new interest, would be to decide on the specific roadnames and era you might like to model, and then to research those specific, and presumably more limited, items online on a manufacturer-by-manufacturer basis. That would certainly be a less intimidating approach.