I haven't had the pleasure of fishing Oil Creek, but it does appear to share some characteristics with some of the other medium-to-large "marginal" or "recovering" trout streams in the state. Like the Tulpehocken and the Little Juniata, caddisflies are very important, but most of the classic early season mayflies are not (Hendricksons, quill Gordons, blue quills). In some ways, this simplifies early season choices until the more tolerant and adaptable "sulphurs" make their appearance.
I don't see B. numerosus listed for your area, but the very similar B. solomoni is listed (along with B. appalachia, lateralis, and nigrosoma). All of these "grannoms" should comprise some significant early season opportunities on Oil.
I'll second Brett's observations on fishing them. (He fishes numerosus, but the information is applicable to most grannom species.) The color change from freshly emerged adults to "aged" adults is something I've mentioned in another thread, and you can see a good example of this by turning to the Brachycentrus appalachia page and comparing the color of the freshly emerged adult (underwater shot) to the aged adult. During the actual emergence, I usually find a good pupa pattern to be the most effective fly, but the grannoms offer better dry-fly opportunities than do many other caddisfly species (especially during their egg-laying activity).
I, too, have had some wonderful experiences with a skittered dry grannom at times (a Henryville Special, Elk-Hair Caddis, or my own Fluttering Sedge in appropriate colors), but it is not a totally reliable phenomenon. Spent or even wet versions of the adults are often more successful. This preference can vary from one day to another, from one spot to another, or even from one fish to another. The rise form will usually tip the fish's hand.
In addition to the variety of grannoms, your area hosts a great selection of Hydropsychidae. Common Ceratopsyche species such as bronta, morosa, sparna, and walkeri, and Hydropsyche species including betteni, hageni, brunneipennis, and scalaris are all recorded for your area. These "spotted sedge" species have tan, cinnamon, or mottled greyish-brown wings and body colors that vary from yellow or yellowish-green to cinnamon or greyish-tan. Again, I find a matching pupa pattern to be best for the emergence, but wet flies (for diving adults) or dry flies (for exhausted or spent adults) can be useful during egg-laying. Due to the prevalence of the Hydropsychidae throughout much of the season, an adult dry used as a searching fly (especially along stream edges and under overhanging vegetation where the adults drop down to drink) can turn up a few fish most anytime.
I'm especially interested in your mention of Nectopsyche. The only species of this genus that I see listed for your area is exquisita, which is one of the more colorful "white millers." Their cream-colored wings are marked with bands of tan and punctuated by four dark spots along the top rear of the (closed) outer wings. Their bodies also tend to be darker (more olive) than the cream or light green bodies of other common PA Nectopsyche. You may also have other Nectopsyche species that have yet to be recorded. I'm curious to know what kind of fishing opportunities these provide on Oil.
The catch with many of the later-emerging Nectopsyche species is that they frequent larger, warmer stretches and water temperatures are not always conducive to trout activity. I suspect that the dwindling supply of stocked trout in Oil could also be a factor with regard to this hatch, but I'd be interested to learn about any successes you've had while fishing this hatch.
I hope this information is useful. Let me know how it gels with your experiences on Oil Creek.