Old Olney is full of Deathclaws. One way to defeat a deathclaw is to start the fight with the deathclaw as far away as possible and using VATS and a dart gun, target and shoot the deathclaw's legs. This will slow the deathclaw's approach and allow you to use any other ranged weapon against the deathclaw to finish the fight. Once Old Olney is cleared of Deathclaws, you can explore the area (including the sewers for more Deathclaws and goods).
Edit: Actually, there is no need to aim at the legs with the dart gun, since one of its properties is that on hit, it cripples both legs (Damage left leg -1000 & Damage right leg -1000). As long as you hit any part (aim at the one with higher percentage), the legs will be crippled. Once the legs are crippled, you can get close to the deathclaws for better hits, just stay out of reach of their attacks.