Oyster mushroom (Pleurotus spp.) cultivation gained a growing concern due to their limited growth requirements and high nutritional value and bioactive compounds. Present work investigates the effect of rice straw supplementation with inorganic P (KH2PO4) and organic P (wheat bran) with presence or absence of agar layer above straw on mycelial growth of two oyster mushrooms: P. pulmonarius and P. floridanus. Mycelial growth was better with bran than with KH2PO4, particularly without agar. Such a differential effect of P form and agar layer was more evident on P. pulmonarius than on P. floridanus. Mycelial growth mostly exhibited a saturable dose-dependent pattern and increased up to a certain optimal P supply of 2–5 g P/g straw, post which it approached a plateau; only in the no agar-bran culture of P. floridanus a progressive P-dependent increase was found. The time-course of fungal growth exhibited contrasting patterns in the two fungi; being sigmoidal in P. pulmonarius but almost linear in P. floridanus. In P. pulmonarius, the lag growth period (T10) was longer with KH2PO4 than with bran, with different dose-dependent patterns in the two P sources and marginal effect of the agar layer. Relative growth rate (RGR) of P. pulmonarius was higher than that of P. floridanus, with different dose-dependent patterns according to fungal species and presence of agar. In P. pulmonarius, both T10 and RGR attained comparable values of about one day and 0.4 day-1, respectively at 5 mg P/g straw for the different P form × culturing mode combinations.