In OSE, and I'm assuming B/X, which it is reorganized from, XP is gained thusly:
"All characters who make it through an adventure alive receive experience points (XP), awarded by the referee. XP is gained from two sources: treasure recovered and monsters defeated."
NPC's, merchants, and farmers don't fit any of those categories, technically. So, yes, one could extrapolate out the 1GP=1XP and apply it to every situation, but no one that I have ever played with has done that. NPC's have levels as necessary for world building or story or from the adventure being used. Only characters gain XP and only when they have returned to "civilization" with that loot. We never played that you go the XP as soon as you found it in the dungeon. For us, it brought up the conundrum of how much we carry with us for XP purposes balanced against being encumbered and thus potentially encountering more random encounters. And our random encounters were never "level adjusted". The plains encounter table had a dragon on it, and it didn't matter what level you were when it came up...
I will echo some of the other posters that the overreliance on making or calling for rolls in games is what causes characters to feel like newbs. The more you roll, the more likely you're going to get a bad result, even on what should be a simple thing to do. We stopped with all rolls except for those with consequences, or those under time pressure. Picking a lock? If you fail, you can try again. But the first attempt took a turn. The next attempt takes a turn. etc. Time wasted. Noise potentially made. More random encounter rolls. Now, we play OSE and adjacent games, so time pressure, random encounters, and managing light sources are a big part of our fun. But we can also go through entire game sessions with few to no rolls being called for, and make amazing progress. YMMV, of course.