Thompson on Wills (2d Ed.) § 375, @ 460, points out that a condition which must be fulfilled before an estate can commence is a condition precedent, while a condition subsequent does not prevent the vesting of an estate, but may serve to defeat a vested estate if the condition is not performed. To this same effect see Wellsville Oil Co. v. Miller, 44 Okla. 493, 145 P. 344; Franklin v. Parks, 77 Okla. 280, 188 P. 334; Fraley v. Wilkinson, 79 Okla. 21, 191 P. 156; Gypsy Oil Co. v. Escoe, 126 Okla. 3, 258 P. 906. In re Assessment, etc., Omitted Property of Kennedy et al., 177 Okla. 74, 58 P.2d 134; In re Clark's Estate (Cal.) 61 P.2d 73; 62 A. L. R. 589; 69 C. J., Wills, § 1783, et seq.

It is next contended that, since the contract provided "that it is the intention of the parties hereto that the said note shall not be due until the final judgment in the said suit to be brought, or defended, by second parties in behalf of first party," that the foregoing provision rendered the claim to the fee agreed upon contingent upon the happening of the event, and that, since the event provided did not happen, intervener could not maintain an action therefor prior to such condition being fulfilled. We are cited to Franklin v. Parks, 77 Okla. 280, 188 P. 334; 12 Am. Jur. 849, § 296; Cole v. Addison, 153 Or. 688, 58 P.2d 1013, 105 A. L. R. 897; Thomson v. Kyle, 39 Fla. 582, 23 So. 12, as supporting the contention so made. An examination of the cases and authorities so cited will reveal that they are wholly without application to the facts involved in the case at bar. A party to a contract may not by his deliberate act prevent the happening of a condition therein and then take advantage of the condition to defeat liability upon the contract.

The following definition of a "condition precedent" has the support of many authorities: "A `condition precedent' is one that is to be performed before the agreement becomes effective, and which calls for the happening of some event or the performance of some act after the terms of the contract have been agreed on, before the contract shall be binding on the parties." ( Mumaw v. Western Southern Life Ins. Co., 97 Ohio St. 1, 119 N.E. 132, 133, 135; Rogers v. Maloney, 85 Or. 61, 165 P. 357, 358; Yerger v. Simmons, 136 La. 280, 67 So. 3, 6; Sunshine Cloak Suit Co. v. Roquette Bros., 30 N.D. 143, 152 N.W. 359, 362, L.R.A. 1916E, 932; Metropolitan Life Ins. Co. v. Goodman, 10 Ala. App. 446, 65 So. 449; Northwestern Nat. Life Ins. Co. v. Ward, 56 Okla. 188, 155 P. 524, 526; Lilly v. Haynes Co-op. Coal Min. Co., 50 N.D. 465, 196 N.W. 556, 559; Franklin v. Parks, 77 Okla. 280, 188 P. 334, 335, and many others.) The following definition of a "condition subsequent" is accepted in substance by most authorities: "`Condition subsequent' operates upon the estate already created and vested, rendering it liable to be defeated, if condition is broken."

A unilateral offer, dependent upon fulfillment of a condition precedent, becomes binding once the condition precedent is fulfilled. See, e.g., Worms v. Burgess, 1980 OK CIV APP 1, ¶ 13, 620 P.2d 455, 458. So, where the obligations of a unilateral contract depend upon the occurrence of some future event, the unilateral contract cannot be enforced unless or until the future event in fact occurs. See, Rollins v. Rayhill, 1948 OK 83, ¶ § 14-16, 200 Okla. 192, 191 P.2d 934, 937-938; Sam P. McCullough, Inc. v. Doggett, 1936 OK 131, ¶ 0(4), 54 P.2d 184; Franklin v. Parks, 1920 OK 106, ¶ 0(1), (4), (6), (7), 188 P. 334. ¶ 14 However, a unilateral contract may ripen into an enforceable bilateral contract upon partial performance.

The court's holding that it was not such a contingent claim as contemplated under the section could not be construed that the court was determining generally that it was not a contingent claim. Other cases which are more or less supporting for our conclusion are the following: In re Littleton'sEstate, 129 N.Y. Misc., 845, 223 N.Y. Supp., 470; In re Concklin's Estate, 150 N.Y. Misc., 53, 268 N.Y. Supp., 348; In re Suderov's Will, 153 N.Y. Misc., 214, 274 N.Y. Supp., 621; Howard, Trustee, v. Swift, 356 Ill. 80, 190 N.E. 102; Chicago Title Trust Co. v. Fine Arts Bldg., 288 Ill. 142, 123 N.E. 300; Bocock, Admr., v. Leet, 210 Ill. App. 402; S. Milwaukee Co. v. Murphy, 112 Wis. 614, 88 N.W. 583; Terhune v. White, Exrx., 34 N.J. Eq. 98; Dunn v. Sublett, Admr., 14 Tex. 521; Franklin v. Parks, 77 Okla. 280, 188 P. 334; 2 Words and Phrases (3 Series), 410. Throughout the consideration of this case we have had in our minds the query as to whether the trial court should not have overruled the demurrer and entered a declaratory judgment, even though the effect of such judgment would have been a determination that plaintiffs could have no case for affirmative relief.